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Introduction: Chestnut (Castanea sativa) trees are generally distributed in the northern part of Iran. Chestnut trees provide eaten fruit and good-quality timber in many countries. Dried chestnut wood has a high resistance to decay due to extractive compounds, such as tannins. Chestnut wood is used for many applications, such as the construction of buildings and wooden furniture, shipbuilding, timbers, and musical instruments. This important tree species was used extensively until the occurence of the fatal disease chestnut blight (CBD) caused by Cryphonectria parasitica, which is a tree pathogen notable for the disease commonly known as chestnut blight, which primarily affects species of sweet chestnut including the American sweet chestnut (Castanea dentata) and European sweet chestnut (C. sativa). The pathogen has been referred to as the chestnut blight fungus. Castanea sativa has a ring-porous wood with larger early wood vessels formed in the spring. The aim of this study was to determine and compare the anatomy and the xylem vessels diameter and some ray attributes of diseased chestnut wood and healthy chestnut wood in the north of Iran. Material and Methods: The sample tree was 20 m in height and 15-40 cm in diameter. The tests specimens were taken at 1.30 m height above the ground in the form of a disc that includes the infected area. In this study the effect of the Cryphonectria parasitica was investigated in wooden and bark samples from healthy and diseased trees in three diameter classes (small medium and large). Both diseased and healthy wood samples with dimensions of 2 x 2 x 2 cm were taken from a diseased chestnut tree to determine the anatomical features of thin (8-15 μm) transverse, radial, and tangential sections. This was done by using a microtome (Leica 820, Germany) for the light microscopical study of wood anatomy. All of the anatomical sections were prepared as recommended by Parsa pajouh and Schweingruber (1988). Then, some thin transversal section were prepared and after different stages of staining, microscope slides were prepared. To achieve statistically reliable results, the guidance provided by The International Association of Wood Anatomists Committee - IAWA Committee (1989) was used to determine the Tangential and Radial diameter of the Spring and summer vessels, and the width, height and number of rays in both healthy and diseased wood .To analysis of data used Anova and t-student tests and for compare mean Duncan test by Spss software. Results and Discussion: Table 2 and 3 provide the average values of the radial and tangential diameter of the healthy and diseased wood vessels and frequency of the rays with their average of width and height. In both of the healthy and diseased parts of the chestnut wood, ring porosity was present, and the annual rings were distinct. The porosity and distinctness of the annual rings were not changed in the diseased wood. However, the tangential diameter of the vessels was 71.93 μm in the summer wood of the diseased part and 65.04 μm in the summer wood of the healthy part. These diameters for the spring wood were determined 457.25 μm and 337.21 μm, respectively. The tangential and radial diameters of the vessels were smaller in the healthy wood than in the diseased wood. The average vessel frequency in the springwood of the diseased and healthy parts were 6 and 6.2, respectively. The vessel frequency values in the summer wood were determined to be 44 and 29, respectively. The results showed that there was significant difference between summer vessel diameter in the three classes of diameter (large, medium and small diameter) at probability level of 5%, in diseased and healthy wood. Whereas the diameter of the radial and tangential spring vessels in diameter classes had no significant difference between medium and low diameter. The irregularities were observed in cross-section due to influence of fungi on structure of wooden fiber in chestnut, the fungi attacks wooden fiber more than other organs of trees. The irregular cell structure of the vessels and fibers of the diseased wood showed (Fig.5) some similarity with cancer in animals and humans. Conclusion: This is the first detailed study that has been conducted on Cryphonectria parasitica diseased Iranian Chestnut trees. One way to prevent the spread of the disease in forests is to remove the diseased trees. These logs can be used in making furniture, shipbuilding, wooden building construction, and musical instruments. Heat treatment can be suggested for more dimensional stability and especially for sterilization of the timber obtained from the diseased trees.