Oxygen contamination of samples fabricated by very high_frequency plasma_enhanced chemical vapor deposition with the variation of silane concentration and disc harge power was studied by secondary ion mass spectroscopy. The results showed t hat oxygen content in the samples depended strongly on the silane concentration and discharge power. Microcrystalline silicon thin films with higher crystallin e volume fraction has relatively higher oxygen content. Oxygen contamination of samples was also related with the background vacuum, especially for microcrystal line silicon thin films. Therefore, higher background vacuum is extremely necess ary in the fabrication of high-quality microcrystalline silicon thin films.