High quality sub microcrystalline diamond films have been synthesized in low temperature via glow plasma assisted hot filament chemical vapor deposition (GP CVD). In situ optical emission spectroscopy is applied to diagnose weak signal of GP CVD system when CH 4 and H 2 are used as the input gases, and reacting particles are identified for diamond growth process. The result shows that the substrate temperature for synthesizing high quality diamond films by EACVD has been reduced from 850 ℃ to (340±5) ℃ by adopting GP CVD. From experimental results, It was found that the existence of a great deal of super saturation atomic hydrogen is the key factor for producing diamond films of high quality, and the main energetic particles are CH 3, CH, CH +, H * etc. in the process of diamond films growth. Glowing of near surface may form a dipole layer, which plays a leading role in low temperature synthesis.