Low-temperature epitaxy (LTE) of crystalline silicon in the plasma-enhanced chemical vapor deposition (PECVD) was investigated using in situ observation of reflection high energy electron diffraction (RHEED). Source gases of SM, and Hz mixture were decomposed by radio-frequency (RF) glow discharge, and an epitaxial layer was grown on a p-type Si(001) substrate at a variety of hydrogen dilution ratios, R{([H-2]+[SiH4])/[SiH4]}, ranging From 10 to 200, and at substrate temperatures, T-s, ranging from 27 degrees C to 560 degrees C. Critical thickness of epitaxy above which polycrystalline growth starts to occur shows two peaks as a function of growth temperature at 120 degrees C and 430 degrees C. The lower temperature peak appeared at 120 degrees C and the surface has a 1 X 1 structure covered by SiH2, while at the higher temperature peak of 430 degrees C the surface has a 2 X 1 (1 X 2) double domain structure along dimmer rows covered by SiH. In addition, at 430 degrees C, RHEED intensity oscillation with an oscillation period of monolayer was observed. This implies a layer-by-layer growth in the PECVD, and suggests a presence of the surface diffusion of Film precursors on the hydrogen-terminated surface. These results are explained in terms of the hydrogen-mediated enhancement of crystal growth corresponding to surface hydride mode.