We have grown extremely thin, single-domain 3C-SiC films by forming a low-temperature interfacial buffer layer using monomethylsilane (H₃C-SiH₃) on nominally on-axis Si(001) substrate, whose miscut is estimated to be less than 0.1−0.2^o. We have clarified that single-domain 3C-SiC(001) 3×2 films can be grown on both Si(001) 2×1 single-domain and Si(001) 2×1 + 1×2 double-domain surfaces. The film is as thin as 45−200 nm, which is compared to the film as required in a previous study (> 5 μm) to achieve the single-domain 3C-SiC films on a nominally on-axis Si(001) substrate. The development of the single domain observed is understood in terms of electromigration during dc-resistive heating and unique adsorption nature of the monomethylsilane.