We have developed a new method for epitaxial growth of ultrathin (similar to nm) 3C-SiC films on Si(100) by pulsed supersonic free jets of methylsilane (CH3SiH3). It was found that pit formation at the SiC/Si(100) interface was suppressed by increasing the pulse width and the surface roughness was decreased by decreasing the number of CH3SiH3 jet pulses. A linear relationship was observed between the film thickness and the pulse number in the thin film region of less than approximate to 40 nm, while the growth rate was decreased and the thickness was eventually saturated for further pulse irradiation.