Degradation behavior of ZnO-glass fiber-unsaturated polyester composite under exposure to a metal halide lamp is investigated by means of DSC and FTIR. The FTIR results show that the UV photons can increase the carbonyl group on the surface, but ZnO can decrease the carbonyl content and further contribute to the breaking of single bond of carbon to oxygen on the surface. All of the results indicate that for pure unsaturated polyester under the UV exposure in air, the major reaction is photo-initiated oxidation, whereas for the composites with ZnO, decarbonylation is the major effect. In addition, when the content of ZnO is not higher than 4 wt %, the composites are still transparent, and its effects on the crosslinking process and thermal stability are insignificant. Thus, ZnO can be used as a stabilizer of unsaturated polyester for the glass fiber reinforced polymer industry. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2128-2133, 2009