Abstract: The Raman scattering spectrum has been used to measure the yield of singlet oxygen molecules in singlet oxygen generator of COIL. We presented the experimental results on a 0.1 mol/s JSOG. The effect of dilution gas on the yield has been studied. The different dilution gas and different dilution ratio have different efficiency to the delta oxygen yield. For a given COIL device, it is essential to choose suitable dilution gas and dilution ratio.