The　relationships　between　the　diffuse　reflectance　of　tissue　and　its　optical　parameters　(especially　the　refractive　index　and　the　ratio　N＇　of　effective　scattering　coefficient　to　absorption　coefficient)　are　studied　by　Monte　Carlo　method.　The　limitation　in　the　diffuse　reflectance　formula　fitted　by　others　is　found,　and　a　new　diffuse　reflectance　formula　which　is　different　for　N＇　in　different　region　is　presented.　And　the　diffuse　reflectance　formulas　are　obtained　for　N′　=2-20　and　N′　=40-100,　respectively.