There is a growing requirement to use supersmooth surfaces with roughness in the sub-nanometer range. But, to produce 100mm-diameter optical elements with ultra-flat and supersmooth surfaces is still difficult. The fabrication technique based on continues polishing process is presented to produce flat optical element with extremely smooth surface. During the fabrication, A concept of "Process Controlling" is introduced, which means the machining of super-smooth surfaces is considered as a chain consisted of some key nodes, not merely a polishing process. The surface figure is tested using interferometer and the surface roughness is using interference microscopy and atom force microscopy (AFM) repectively. Then the Power Spectral Density (PSD) function, including the basic theory and the physical meaning, are presented to explain the difference of test results, which is measured by optical profiler and AFM with different parameters. The polynomial fitting results indicate that there is excellent agreement between measurements made by the two instruments.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.