Abstract: In order to study the frequency up-conversion in single model double cladding Er3+-Yb3+co-doped fiber,the green fluorescence emitted by single model double cladding Er3+-Yb3+co-doped fiber was measured with a fluorphotometer and compared with the green fluorescence in Yb3+ doped fiber.The results show that the fluorescence in the single model double cladding Er3+-Yb3+ co-doped fiber is attributed to the absorption of Er3+ ions in the excited state of and the Yb3+ ions only transfer the pump energy to the Er3+ ions.It leads to the difference of emissive fluorescence between the double cladding Er3+-Yb3+co-doped fiber and Yb3+ doped fiber in the spectra and the power and its variation law.