Initial growth stage of 3C-SiC, which was initiated by exposing a Si(001)-(2×1) surface to dimethylsilane (DMSi) at substrate temperature 650−750^oC, was observed in situ using reflection high-energy electron diffraction (RHEED). From the observation of RHEED patterns, 3C-SiC spots appeared after incubation time (IT). During IT, Si(001)-c(4×4) structure was also observed. These changes were similar to the case using cracked propane. Therefore, it is considered that the carbon atoms from adsorbed DMSi molecules diffused into the Si substrate and formed Si_1-xC_x alloys in oder to relax the lattice mismatch between Si and 3C-SiC during IT. The activation energy calculated from the Arrhenius plot on the initial growth rate of 3C-SiC formation was close to that for hydrogen desorption from Si surface. The hydrogen atoms of DMSi molecules adsorbed on the surface transferred to the dangling bonds and afterwards they desorbed from Si-H bonds on the surface. The hydrogen desorption process is considered to be rate-limitting step.