This paper studies the dynamic analysis and characterization of two-stage planetary gear transmission system for wind turbine drive train under random loads. Rigid multi body dynamics with discrete flexibility is used for modeling the wind turbine drive train. Variable gear mesh stiffness, bearings and shaft stiffness are accounted in this model. The derivation of the system governing equation by Lagrange approach requires the formulation of kinetic and potential energies. The coupled torsional and bending model of 22 degrees of freedom is solved for vibration responses in time domain using Newmark time algorithm. Applying the Fast Fourier Transformation (FFT) on a time signal, vibration responses in frequency domain are obtained and further studied for wind turbine drive train characteristics. To identify the wind characteristics at low frequencies, wavelet decomposition is used. © 2018 Author(s).