We have conducted a systematic series of gas-source MBE experiments of 3C-SiC on Si(100) using monomethylsilane (MMS), and have investigated the relation between growth parameters (MMS pressure and growth temperature) and the grown film quality using atomic force microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Auger electron spectroscopy. As a result, it was clarified that there exists a set of optimum growth parameters for the best surface morphology and crystallinity. The optimum temperature lowered with decreasing MMS pressure, and the crystallinity of the SiC film improved at the same time. In particular, a high quality 3C-SiC film on Si(100) was successfully grown at T = 900 degrees C.