Abstract: The light stabilizer/di-methyl silicon oil blend composites were prepared by adding hydroxy-4-methoxy-benzophenone (UV-9) and a hindered amine light stabilizer named GW-783 into di-methyl silicon oil and the composites applied as heat transfer medium for concentrator solar cells were studied. By accelerated life test, the weather resistance of the composites, viscosity and spectral transmittance were investigated and the mass fraction range of light stabilizer in composites during working process were defined. And the performance of concentrator soalr cells before and after test was also detected and analyzed. The results show that the aging effect on light stabilizer/di-methyl silicon oil blend composites can be inhibited efficiently and there are no interaction between concentrator solar cells and composites. Hence, the power performance of concentrator solar cells can be kept stable when concentrating photovoltaic (CPV) system is running for a long time. The most effect composite is UV-9-GW-783/di-methyl silicon oil.