This paper deals with the analysis of the performance of different wind turbines using the Similitude Theory. Wind turbine performance was determined as a function of geometrical similarity coefficient, which is related to all parameters of the Similitude Theory. Therefore, a mathematical model simplification is possible in the ‘in similitude’ wind turbines comparison. The mathematical model for wind turbine performance is based on BEM Theory, and its efficacy was verified several times by comparing different wind turbine experimental data. The original mathematical model was modified to take into account Similitude Theory parameters. The model is able to determine which wind turbine is most suited to particular design specification. This work presents power and torque curves, power and torque coefficients as functions of rotational speed and wind velocity. All the results are function of the geometrical similarity coefficient. With this methodology it is possible to maximize the power coefficients of a wind turbine, and it is possible to identify a family of wind turbines, geometrically different, but with the same high performances.