Abstract: The DC53 steel after conventional heat treatment was subjected to cryogenic treatment in the −196 ℃ liquid nitrogen for different time periods (3, 6, 9 h). End milling and side milling were carried out by using cemented carbide end mills. The effects of cryogenic treatment on the microstructure and milling machined surface quality of the test steel were studied. The results show that the tempered martensite and retained austenite in the DC53 steel were refined after cryogenic treatment. The amount of fine granular carbides increased and dispersed in the matrix. With the extension of cryogenic treatment time, the grain refinement effect became more obvious, the amount of fine granular carbides increased, and the microstructure became more uniform. When the cryogenic treatment time was extended to 9 h, the size of some granular carbides increased. After cryogenic treatment, the number and depth of grooves or furrows on the milling machined surface decreased, and the surface roughness was reduced. As the cryogenic treatment time extended, the number and depth of grooves or furrows first decreased and then slightly increased, and the surface roughness also first decreased and then slightly increased. After cryogenic treatment for 6 h, the milling machined surface roughness was the lowest, and compared with those before cryogenic treatment, the surface roughness of end milling and side milling machined surfaces decreased by 25.8% and 21.6%, respectively; the surface quality was the best.