The wind turbine drive train dynamic analysis is carried out with and without existence of gear tooth breakage. The loads acting on the wind turbine are considered based on Danish Standard DS472 which is a simplified method of taking loads in wind turbine. A broken gear tooth is modelled by the change in the gear tooth thickness. The gear mesh stiffness of broken tooth is evaluated by the change in cross sectional area and area moment of inertia due to change in tooth geometry. From the frequency representation of vibration data, the fault information is not clearly observed due to the insensitiveness of the dynamic system to the stochastic loads. However, by further analysis, fault information is obtained with the use of the gear tooth contact force from which a clear sub harmonics are observed at the defective pinion tooth frequency. © Springer International Publishing Switzerland 2015.