Planetary gear systems are widely used in wind power systems because of the advantages of compact design, large carrying capacity, and high transmission efficiency. Despite these advantages, the several conditions under which such gears are typically used may lead to failure. Tooth pitting is one of the frequently encountered failure modes. The damage of teeth (as tooth pitting) is modeled for a wind turbine drive train. The drive train has got a three-stage gearbox that contains two high-speed parallel gear stages and a low-speed planetary gear stage. The proposed dynamic model of a wind turbine also includes a rotor and an electric generator. The model consists of 10 bodies and has got 11 degrees of freedom. The aerodynamic and generator torques are applied as external loads. The influence of tooth pitting on the gear mesh stiffness is scrutinized. Healthy drive train dynamic response and response of drive train containing tooth pitting are compared. The obtained results give an useful information about health monitoring of wind turbine drive train.