A novel Si epitaxial growth technique using mercury‐sensitized photochemical vapor deposition has been developed. Epitaxial thin films (300–8000 A˚) were grown on (100)Si substrates at 100–300 °C from a gas mixture of Si2H6+SiH2F2+H2by irradiation of a low pressure mercury lamp (1849,2537 A˚). The growth rate, plotted as a function of the reciprocal substrate temperature, represented an activation energy which was found to be a small value of 0.18 eV. Observation of the surface structure by reflective high‐energy electron diffraction showed fine (100) streak patterns for films grown at 200 °C as well as at 250 and 300 °C.