A stochastic uncertain tribodynamic model is established for a spur gear pair for the first time. The stochastic uncertainty of pinion rotation speed propagated to lubrication performance is investigated. The probability density function of the minimum lubricant film thickness h_min evolves over time periodically at interval of an engagement process. Correspondence between abrupt increase in meshing force and amplification of h_min uncertainty is found. Robust and reliable lubrication performance can be achieved by suppressing the h_min uncertainty and decreasing the lubrication failure probability. This can be done by increasing lubricant viscosity, and decreasing input torque and uncertainty level of input rotation speed. This work lays a solid foundation for robust and reliability based optimization for tribodynamic gear system.