The micropattern of SiO2 film was prepared by the photooxidation of an oganosilicon nanocluster which consists of a three-dimensional Si-core and organic groups on the surface. By UV-irradiation in air, the organosilicon nanocluster film became optically transparent in the UV-vis region. As an intermediate during the photobleaching, a silyl silylene was observed by laser flash photolysis of organosilicon nanocluster. The FT-IR spectra showed the conversion of Si-Si bond to Si-O-Si bond and the elimination of organic substituent during the photooxidation. The XPS spectra of Si 2p region for the photooxidized film at room temperature showed a peak assigned to SiO2 at 103.4 eV. This is in contrast to a quasi-two-dimensional network polysilane which gives a peak of siloxane structure by photooxidation. The difference was explained by considering the three-dimensional Si-Si network structure of onganosilicon nanocluster. The TGA analysis in air showed a rapid weight loss at 164 degrees C, and the weight remained nearly constant up to 1000 degrees C after the thermal oxidation. The micropattern of SiO2 was formed by the UV-irradiation of the organosilicon nanocluster film with photomask and the development using an organic solvent. The transparent micropattern showed excellent dimensional stability up to 500 degrees C.