We have conducted a systematic series of 3C-SiC/Si(100) gas-source MBE experiments using monomethylsilane (MMS: H3Si-CH3), and have obtained growth-temperature and MMS-pressure dependences of the grown film. As a result, an optimum growth temperature was found to exist for each pressure, which decreased with decreasing pressure. The low temperature degradation of the film is related to residual surface hydrogen termination as suggested from H-2-temperature-programmed-desorption observations from MMS-adsorbed Si(100) surfaces. The high temperature degradation is understood by the onset of Si outdiffusion from the substrate into the film. A balance between these two competing processes accounts for the presence of the optimum growth temperature and its pressure dependence as well.