The process of most machine loading has a random character which is determined by external varialion and dynamic qualifies of the system. Such an approach was used to obtainthe load spectrum for gears and the probability of its failure. Analysis and experiments showed that gain factor for gear is distinguished by the similar parameter of the entire drive system. This is explained by the uniform distribution of gear stress even for cases of smile loading. Method of gear strength calculationwith the statistical parameter is discussed.