Analysis of transmission error curve of a pair of gears measured by a single flank meshing tester shows that the error can be separated into harmonic components and random ones. In this paper the effect of the random components of the error on dynamic loads is analysed theoretically and one example of numerical result is shown. Dynamic behavior of gear systems is considered as nonlinear, nonstationary random process because of variable spring constant, backlash term and random component of error, so the Fokker-Planck equation is derived. It is analysed approximately using statistical linearization and the mean and the variance of dynamic loads are calculated numerically. Monte-Carlo-Simulation is also performed to find dynamic loads by means of an analogue computer. The results show that the mean of the dynamic loads remains at similar level as in deterministic process where the random components of error are not considered for any gear speed, but the variance of them increases rapidly with an increase in speed over resonance frequency.