Purpose / Context - Cool coating is a passive technique for reducing the solar heat gain and cooling energy consumption of buildings. However the radiation properties (i.e., solar reflectance and thermal emittance) of cool coating would be changed, usually adversely, during its ageing process, and its thermal performance would be degraded accordingly. UV could cause photolysis and photo-oxidative reactions in cool coating, thus is believed to be one cause for the degradation. Methodology / Approach – Groups of cool coating samples were exposed to UV radiation in an accelerated ageing chamber. The effects of UV were quantified by comparing the radiation properties of the samples before and after the exposure. Furthermore, the results of this UV exposure experiment were compared to a nature exposure experiment, to figure out the contribution of UV in the whole ageing process, which comprises the effects of numerous factors. Soiling is an important factor in the ageing process, thus its effects were investigated in this UV study as well. Key Findings / Implications – UV was found to be responsible for several percent of the total loss of solar reflectance of cool coating; while soiling was found to be the most significant factor for the total loss in the ageing process. Originality - Few studies have been conducted on quantifying the UV effects on the changes of radiation properties of cool coatings. This study fills this gap.