By analyzing the rock fragmentation mechanism of diamond in diamond bit, the rock cutting depth with diamond, the area of diamond pressing into rock, the load on each diamond and the pressure on rock from diamond are calculated. After analyzing the result of calculation, it comes to a conclusion that volume crushing contributes most in rock fragmentation process of diamond bit. The method of choosing diamond quality in diamond bit is also explained as well. Selecting the quality of diamond plays a vital role in product performance. The effectiveness of diamond bit is involved with not only the density and particle size of diamond, but also the quality of diamond. Based on analyzing the rock fragmentation mechanism of diamond, this paper researches on selecting diamond quality.