The life of tri-cone bits will directly affect the economic and technical drilling target. In order to control and evaluate bit’s life, the paper builds up the calculating mathematical model, which is suitable for determination of tri-cone bit’s life, to rationally design the radial clearance of non standard bearings. Effect of the radial clearance on the life of roller bearing was systematically deduced. The larger the radial clearance is; the higher the contact stress is; and the shorter the roller bearing life is. It follows from the model that the key factor of the radial clearance should be taken into consideration besides structure data and anti-friction pair’s heat treatment of roller bearing in order to decrease the contact stress of anti-friction pair and prolong the bit’s life.