Wind power assessment in complex terrain is a very demanding task. Modeling wind conditions with standard linear models does not sufficiently reproduce wind conditions in complex terrain, especially on leeward sides of terrain slopes, primarily due to the vorticity. A more complex nolinear model, based on Reynolds averaged Navier-Stokes equations have been used. Turbulence was modeled by modified two-equations k- model for neutral atmospheric boundary layer conditios, written in general curvelinear nonorthogonal coordinate system. The full set of mass and momentum conservation equations as well as turbulence model equations are numerically solved, knowing as computational fluid dynamics (CFD) technique. A comparison of the application of linear model and nonlinear model is presented. Considerable discrepancies of estimated wind speed have been obtained using linear and nonlinear models. Statistics of annual electricity production vary up to 30% of the model site. Even anemometer measurements directly at a wind turbine's site do not necessarily deliver the results needed for prediction calculations, as extrapolations of wind speed to hub height is tricky. The results of the simulation are compared by means of the turbine type, quality and quantity of the wind data and load factor. Finally, the comparison to the results with the measured data at 10, 30 and 50m.