The aim of the present project is to get a better understanding of the stability properties of wakes generated by wind turbine rotors. To determine the stability properties of wind turbine wakes a numerical study on the stability of the tip vortices behind the Tjaereborg wind turbine has been carried out. The numerical model is based on large eddy simulations of the Navier-Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies the flow is perturbed by inserting a harmonic perturbation. The results show that the instability is dispersive and that growth arise only for some specific frequencies and type of modes.