The cryogenic treatment is an extension of the cold processing technology by which the material properties could be changed. This provides a new method for people to improving the performance of the cutting tools and prolonging the tool life. The micro-structure of two unknown types of carbide reamers after different cryogenic treatment was introduced in this paper. From the metallographic microstructure, SEM microstructure, EDS spectrum analysis and the hardness measurement, the detailed analysis on the microstructure of the reamers (1# and 2#) was provided in this paper. The results show that the matrix materials of the two reamers were the WC-Co cemented carbide, and they were the ultra-fine grain carbide (＜500nm); 2# reamer belonged to the high Co cemented carbide reamer. By the cryogenic treatment, the hardness of two reamers has increased. And the value of the hardness for 1# reamer was higher than that for 2#.