Investigation on very high frequency (VHF) plasma and its glow discharge mechanism during the deposition of microcrystalline silicon (muc-Si:H) film have been investigated. In situ optical emission spectroscopy (OES) measurements were used to study the influence of deposition conditions on VHF plasma. The study reveals that: the OES intensities of Si*, SiH*, Halpha * and Hbeta * increase significantly with plasma excitation frequency; all the OES intensities increase initially with plasma power and tend to be saturated at working pressure 120 Pa, a maximum can be observed while at 60 Pa; the variation of the intensities with working gas pressure strongly depends on plasma power, and maximum intensity for every peak was observed between 100 and 120 Pa. Based on the analysis of the OES spectra, the glow discharge mechanism and the relationship between the OES spectra and the deposition rates are also discussed. (C) 2004 Published by Elsevier B.V.