The same batch of clinker was ground by roller mill, ball mill, vibratory mill and disk mill respectively in laboratory. The test results show that the strength of cements ground by different mills is obviously different. With similar specific surface area of cement, the strength of roller mill cement is the highest, next is the ball mill cement, the third is the vibratory mill cement and the strength of the disk mill cement is the lowest. While the cement was ground by the same mill, the finer the cement, the higher the strength. The reasons for strength difference are analyzed from the aspects of cement particle size distribution, mineral compositions in each particle grading and cement hydration degree.