Roughness is one of the most important characters of optical surfaces, especially in the case of diamond turned surfaces, the surface roughness reflects the dynamic characters of the diamona turning machine. It is suitable to monitor the roughness of diamond turned surfaces by light scattering. But up to now almost all of the scattering methods only deal with the flat surfaces. In a lot of actual cases, e.g. for machining the x-ray microscope objectives or Synchrotron Radiation beam-line mirrors, it is necesstrry to treat the surfaces with definite form. In this paper the author set up a theoretical model for describing light scattering from formed surfaces, and by means of statistical analysis of the scattered light, the contributions of form and roughness to the angular distribution of light can be separated, and the influences of the surface form on the testing of the surface roughness by light scattering can be removed.