A new technique wasdeveloped which obtains selectively the htick fully recessed oxidized porous silicon layer (OPSL) with good dielectric property. The porous silicon layer was ocnverted to thick fully recessed oxide (FRO) with 3-step (1${mu}$m, 1.5${mu}$m, 1.8${mu}$m) by multi-step thermal oxidation (after 400$^{circ}$C, 1 hour by dry oxidation, 700$^{circ}$C, 1 hour and then 1100$^{circ}$C, 1 hour by wet oxidation). The breakdwon field of the FRO was about 2.5MV/cm and the leakage current was several pA ~ 100 pA in the range of 0 of 90 pF. The progress of oxidation of a porous silicon layer was studied by examining the infrared abosrption spectra. The refractive index (1.51) of the fRO, which was measured by ellipsometer, was comparable to that of the thermally grown silicon dioxide (1.46). The etching rate (1600${AA}$/min) of the FRO was also almost equal to that of the thermal oxide.