Using conbinatin of straight and folded ring down cavity technique, the high reflectivity of COIL mirrors at 1.315μm has been pcisely measured. The experimental results show that the reflectivity is as high as 99.931% with measuring accuracy 10－５.Mirrors measured can be opaque with any size at any angle of reflection. A cavity with lower reflectivity mirrors can be used to measure the higher reflectivity mirrors．