Reflection high-energy electron diffraction oscillations were observed during molecular beam epitaxy of GaAs at temperatures as low as 150 °C under various V/III beam equivalent pressure ratios using As4 as an arsenic source. At the As/Ga beam equivalent pressure ratio of 40, the amplitude of the oscillations was shown to first decrease with the decrease of substrate temperature and then increase when the temperature was further lowered to below 300 °C. The oscillation characteristics at 200 °C were comparable to those recorded at around 600 °C. At fixed temperatures in the low temperature region (<350 °C), the oscillations were shown to be greatly affected by the V/III ratio, with the maximum amplitude at a certain V/III ratio which depends on the substrate temperature used. The strong oscillations at low temperatures and at high V/III ratios were tentatively explained by assuming that the migration of Ga atoms is enhanced on the surface passivated by excess As.