The vibration characteristics of the gear system change when gear tooth crack occurs during operating conditions and the system has to be monitored continuously to prevent the damage. A tooth crack model is developed with the mean gear mesh stiffness for external spur gear pair with different crack propagating lengths. Here, potential energy is used to obtain the analytical expression for the mean gear mesh stiffness and mean mesh stiffness with crack is incorporated into the wind turbine gear box and the dynamic analysis is performed with and without crack. The loads acting on the wind turbine drive train considered here are random in nature. The responses of wind turbine drive train in time and frequency domain are used to study the effects of gear tooth crack. The results showed the sidebands at tooth defective frequency in frequency domain and are found to be more sensitive than the gear mesh frequency and its harmonics. Hence the developed crack model is able to predict the presence of gear tooth crack in vibration response. © Proceedings of ISMA 2018 - International Conference on Noise and Vibration Engineering and USD 2018 - International Conference on Uncertainty in Structural Dynamics. All rights reserved.