This paper presents a decision tool for the optimisation of wind turbine technical parameters, using a multi-physical model of the power system. This includes a multi-physical modelling of aerodynamical, mechanical and electro-mechanical system behaviours. The aerodynamics is based on a blade element momentum model. A mass model of wind turbines components is also used in this paper. The optimisation is performed with NSGA-II algorithm which may choose technical parameters (blade length, transmission ratio or electro-mechanical coefficient in this example) to maximise performances indicators (in this example the output electrical power and the wind turbine mass). The results provide a wide range of solutions that are the best compromises between the performances indicators chosen. The diversity in terms of parameters allows great latitude in the design of wind turbine.