In the present work weathering peculiarities for heat-treated wood plastic composites (HTWPC) were investigated mainly focusing on the effects caused by UV radiation. In addition, the discolouration mechanism for WPC was analysed in general. The results showed that the colour change during UV exposure for WPC with untreated wood particles (WPC(UT)) initially is caused by wood particle discolouration, which is followed by rapid increase in lightness that is due to polypropylene photodegradation. For HTWPC initial colour change caused by heat-treated wood particles was insignificant, however eventually the discolouration for the composite was several times larger than for WPC(UT) due to polypropylene photodegradation in combination with darker initial colour of the composite. Microcracking that reduce the light transmittance of the polypropylene matrix was the main reason for the large colour change. Despite the larger discolouration, the results regarding surface erosion and microcracking showed that HTWPC are more resistant against UV radiation.