The cause of the thickness non-uniformity in the large area deposition of SiO2 films by PECVD(Plasma Enhanced Chemical Vapor Deposition) was investigated by the plasma diagnostics. The spatial distribution of the plasma species in the chamber was obtained with DLP(Double Langmuir Probe) and the new-designed probe-type QMS(Quadrupole Mass Spectrometer). From the relationship between the spatial distribution of the plasma species and the depositing rate of the SiO2 films, it was conformed that the non-uniform deposition of SiO2 films was related with the spatial distribution of the oxygen radical density and electron temperature.