Layer-by-Iayer growth of silicon nitride by NH3 and SiH4 was investigated using an ultraclean lowpressure CVD system with a Xe flash lamp. Thermal nitridation on Si(lOO) in an NH3 environment with and without the flash lamp light irradiation is explained assuming Langmuir-type physical adsorption of NH3 and reaction of the adsorbed NH3. The reaction can be enhanced by Xe flash lamp light irradiation, and the N atom concentration tends to saturate to ~2.7×1015cm-2. In Si deposition on the ultrathin silicon nitride, it is found that N desorption from the Si nitride films hardly occurs, and the deposited Si atom concentration increases up to about the single atomic layer concentration (6.8×1014cm-2), and then the Si deposition rate drastically increases. From FT1R/ATR and RHEED measurements, the structures of the Si film on Si and the Si nitride are found to be different. Layer-by-layer growth control of Si nitride is proposed by combining atomic-layer nitridation on Si and atomic-layer growth of Si on the Si nitride.