This study demonstrated the use of quadrupole mass spectrometry and optical emission spectrometry in diagnosing the plasma in the electron cyclotron resonance chemical vapor deposition (ECRCVD) process. The effects of adjusting the main magnetic coil current and process pressure on chemical composition of the plasma and the characteristics of the epitaxial thin film in the ECRCVD system were investigated. When the main magnetic coil current increased, the deposition rate of thin film increased, with no major effect on thin film crystallization. However, when the process pressure was higher, both the deposition rate and crystallization of epitaxial thin film increased.