In high speed application requiring field weakening operation, iron losses could be really significant and have several origins: the frequency linked to the speed and the flux density waveform in the iron. In field weakening operation, the fundamental of flux densities waveforms decreases and could have a mitigating effect on the iron losses. However the harmonic content increases too and could have an accentuation effect on the level of iron losses. This paper performs the study of iron losses in Internal Permanent Magnet Synchronous Machine (IPMSM) during the field weakening operation, which can be characterized by relatively high harmonic content. The calculation is based on Flux Densities Waveforms (FDW) in the teeth and in the yoke. For that case an original and mathematical model was developed in order to provide a fast and accurate estimation of iron losses for the whole operating space of the electrical machine by use nonlinear losses coefficients taking into account the FDW with a polynomial form of iron losses in function of the fundamental electric frequency. The results of the calculations will show the flux density waveforms in the yoke and in the teeth along the harmonic decomposition for two points: without field weakening and at the end of field weakening curve. It is will show that iron losses do not follow the same evolution in the yoke and the teeth by the fact that the flux density distribution in teeth has higher harmonic content during field weakening operation. The paper presents the methodology and shows the approach for estimation of losses during field weakening application.