Methane (CH4) and hydrogen (H-2) reactive gas mixture has been in situ investigated in a hot filament diamond chemical vapor deposition reactor with a negatively variable biasing voltage applied to the hot filament with respect to the substrate using infrared absorption spectroscopy and optical emission spectroscopy. It is found that CH4 converts increasingly to C2H2 upon raising the filament temperature in a pure thermal activation state, no optical emission of species is observed. Upon bias application, both CH4 and C2H2 in infrared (IR) absorption intensity decrease with increasing bias current, even the IR absorption intensity of C2H2 decreases more rapidly than that of CH4. Meanwhile, the clear emission lines indexed to H, CH, and CH+ appear in the optical emission spectrum obtained, showing that a large amount of excited radicals are produced in the gas phase after applying bias. It is believed that the further generation of activated radicals induced by electron impact may be responsible for the increase of the growth rate of diamond films due to bias application.