Abstract: Effects of qucenching and tempering on microstructure of C95 oil casing steel were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results show that the austenite in test steel after direct quenching was elongated as flat shape along the rolling direction, while it was equiaxial after reheating quenching. As the tempering temperature increasing, martensite laths merged, the size of martensite laths became wider, the boundaries of laths were blured, the precipitated carbides were coarsened, and the proportion of low-angle grain boundary between neighboring crystallographic grains decreased in the test steel. The test steel after direct quenching had smaller martensite laths, higher dislocation density and higher ratio of low-angle grain boundary between neighboring crystallographic grains compared to the reheat quenched test steel.