We present the application of quadrupole mass spectrometer (QMS), optical emission spectroscopy (OES), and Langmuir probe (LP) as an integrated technique for plasma characterization of the process during electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to investigate the use of QMS to determine the relationship between plasma and thin film characteristics, in particular microstructure parameter (R*) and hydrogen content (CH). The OES was used to assist the QMS in measuring the light and simple radicals, such as the H, Si and SiH, and the associated physical plasma properties which include electron density (Ne), electron energy (Te) and sheath potential (Vs) would be studied with the Langmuir probe. The effects of the two ECR-CVD parameters, namely the power density (1 ∼ 4 w/cm2) and hydrogen dilution ratio (H2/SiH4, 0 ∼ 25) were discussed. The results indicated that there is a very obvious difference in QMS analysis between plasma and gas, and it is due to the fact that the QMS analysis of plasma is accompanied with the vapor and surface reactions. Therefore, the consumption of parent molecule (SiH4) needed to be considered. The TIMS (Threshold ionization mass spectrometry) method which is usually applied to estimate the relative density trends of the ground-state silane radicals (SiHx, x < 4) was used in this study. It was found that increasing power density and hydrogen dilution ratio would lead to higher electron density and more SiH2 generated in the plasma. Furthermore, when comparing with the TIMS analysis reported by other authors, we found the ratio of SiH2 to SiH3 from QMS could be considered as a better indicator of film quality for R*.