Low temperature and high growth rate selective epitaxial growth (SEG) on Si02 patterned Si (001) substrate in gas-source molecular-beam epitaxy (GS-MBE) using pure Si2H6 has been investigated by RHEED observation. In the temperature range of 550 to 850°C, SEG was completely obtained at an initial growth stage. Limiting conditions of SEG were closely related with critical volume of supply gas that was equal to the total amount molecules supplied on SiO2 surface during the incubation period of initial growth. The surface SiO2 was induced to evaporate with Si2H6 supplied above 800°C, so that thermal cleaning temperature for removing native oxide came down to 800°C. As a result, the maximum process temperature of Si SEG now became 800°C, and its growth rate reached as high as 645A/min at growth temperature of 700°C.