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Abstract: The thermal method is the primary method used to improve the recovery from reservoirs with heavy or extra-heavy oil. However, the efficiency and economic costliness of the traditional thermal process are limited by the unfavorable interfacial properties of heavy oil/water (steam)/rock system. Therefore, chemicals that can change the wettability and reduce the interfacial tension have been added to hot-water or steam to improve the efficiency of thermal recovery methods. This thesis aimed at screening thermal-resistant chemicals as interfacial modifiers to improve the heavy oil recovery and further investigating their working mechanism on the nanoscale. Different types of new chemicals were tested in this study. High pH solution, ionic liquid, surfactants, and nanoparticles are evaluated by their thermal stability, interfacial alteration, and recovery improvement. The stability of the chemicals was tested through TGA (thermal gravimetric analysis) at steam temperature up 400℃. The theoretically optimized concentration that led to the lowest surface energy was achieved by measuring the interfacial tension between crude oil and solutions with various concentrations. The suitability of the chemicals as wettability modifiers for different rock types was evaluated by contact angle measurements at high temperature and high pressure. Wettability alteration mechanism was further analyzed with atomic force microscopy (AFM), which provided the topography change of mica or calcite surface, which illustrated the deposition of the chemicals and removal of the existing oil layer. Imbibition tests were performed on sandstone and limestone cores with screened promising modifiers at 90 and 180℃.