Drive systems are composed of spiral bevel gear, axle and bearings. In this paper, drive systems and the part of them are analyzed and a correlation of the factor that shows the geometry of spiral bevel gear is evaluated. The Weibull distribution of probability for survival, which caused by the load of bearings and gear teeth, would be calculated, and the life and reliability with equivalent function could be measured more specifically. The reliability methods are applied as a probability of which the gear drive systems are satisfiably operated.