Three kinds of SOI (Si on Insulator) samples, i.e., SIMOX (Separation by Implanted Oxygen), ZMR (Zone Melt Recrystallization) and DB (Direct Bond), have been optically characterized by measuring normal incidence reflectance R spectra from 200 to 2000 nm and spectroellipsometric parameters (Psi) and (Delta) from 230 to 700 nm. The sample structure analyzed is as follows; Air/SiO2(d0)/rough interface ((sigma) )/Si(d1)/SiO2(d2)/Si. Using established values for n and k of Si and SiO2, we performed fitting to get values of (sigma) , d1 and d2 with nm accuracy. It was found that (sigma) equals 0 for SIMOX while (sigma) equals 7.7 nm for ZMR. For DB initially (sigma) equals 0, but thinning treatment of top Si layer by intentional thermal oxidization followed by HF etching emerges surface roughness as large as (sigma) equals 3.3 nm. It was also found that the thickness of top SiO2 layer d0 is about 3.3 nm and affects mainly (Psi) and (Delta) while (sigma) affects mainly R spectra. It is shown that a method to calculate the influence of interface roughness using effective medium approximation (EMA), which is widely used in commercial ellipsometers, is not appropriate to explain experimental results.