In this study we attempted to diagnose the states and properties of plasma generated while depositing SiON thin films using PECVD (Plasma enhanced chemical vapor deposition). The temperature and density of electron gases formed in a PECVD chamber were measured by Langmuir probe method. Their values were also estimated under some assumptions we made in this work. Comparison between experimental and theoretical values of the temperature and density of electron gases was made. The experimental and estimated results revealed that, as RF Power gets higher, the electron density linearly increases, but that the electron temperature does not vary.