Microscopic observations performed on the cross section of the joining area reveal that the increase of the laser power and the decrease of the scanning speed lead to a larger volume of the weld zone with a more important depth penetration. The geometry of the weld zone is elliptic. Microscopic Fourier transform infrared (FTIR) spectroscopy method shows that diode laser welding induces thermal degradation of the polypropylene by random chain scissions. An increase of the crystallinity along the cross section of the welding joint was observed with a maximum reached in the centre of the weld zone. Some process parameters produced the occurrence of a void due to the thermal decomposition and the vaporisation of the polypropylene.