The paper proposes a design optimisation of an Interior Permanent Magnet synchronous motors with maximum output power density and suitable for wide constant-power region operation. In this paper, analytical magnetic and electrical models of the machine are developed to calculate parameters and variables of the machine needed for a design optimization such as flux, resistance and inductances. And then, the thermal aspect is modelled using a thermal lumped-parameter network which allows to estimate the machine temperatures at key points such as the windings and the magnet. These models are included in the optimization loop and so are evaluated at each iteration. The optimization method uses a differential evolution algorithm (DEA). Finally, output performances of the designed motor are verified by finite element analysis (FEA).