Silicon oxide films have been prepared by radio frequency plasma-enhanced chemical vapor deposition using only an organosilicon compound as a source gas without mixing oxygen. Five types of organosilicon compounds, namely, tetramethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, and tetramethylsilane were used. Ultraviolet light was vertically irradiated on a substrate during deposition to enhance carbon contamination removal from the deposited films and improve chemical composition (increase the O/Si ratio). Films prepared with ultraviolet irradiation were high-quality with lower carbon contamination. Furthermore, the O/Si ratios of the films increased in the presence of the ultraviolet irradiation; however no effects due to ultraviolet irradiation were observed when the films were prepared with tetramethylsilane, as it does not contain any oxygen atoms. It is clear that oxygen atoms in the organosilicon compounds were enhanced by the carbon contamination removal from the films and the increase of the O/Si ratio when ultraviolet light was irradiated vertically on the substrate. However, the chemical bonding states changed very Little when ultraviolet light was irradiated horizontally on the substrate. We applied this technique to plasma systems not using oxygen gas and succeeded in decreasing carbon contamination and increasing the O/Si ratios in the deposited films. The microhardness of the deposited films increased remarkably with increasing numbers of methoxy groups and with vertical ultraviolet irradiation.