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Abstract: The microstructures of lath martensite in 25CrMo48V steel quenched at 900°C and tempered at 600–700°C for 5–105min were characterized by field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The tensile properties of 25CrMo48V martensitic steel were evaluated from the specimens subjected to various tempering temperature and time. The result shows that with the tempering temperature and time decreased, the refinement of martensitic packet, block and lath can take place and induce the enhancement of the yield strength. The yield strength follows the classical formula of Hall–Petch relationship with the packet size and block width, and the value of ky is 102.7Nmm−3/2 and 46.6Nmm−3/2 respectively. The yield strength is more dependent on the packet size than the block width, while the martensitic lath is the minimum structure unit controlling the yield strength of the steel. [Copyright &y& Elsevier]