A study on a GaAs buffer layer on a GaP (100) substrate in the two-step molecular beam epitaxial growth is described. In this growth technique, the GaAs buffer layer grown at a low temperature plays an important role in accommodating the lattice mismatch. We examine the influences of the growth conditions on the structural properties of the layers by varying growth temperature, As/Ga flux ratio and annealing temperature. The surface of the grown GaAs layers were amorphous-like for the growth temperature less than 125°C and As/Ga flux ratio over 3, and were crystalline for the temperatures higher than 150°C and the flux ratio of 3, or for 100°C and the flux ratio of 1. After being annealed at 500°C, the lattice mismatch of the former was accommodated perfectly, but the latter was not accommodated. AES analysis showed that an excess As was incorporated in the GaAs layer with the amorphous like surface, and the excess As was desorbed from the layer during the annealing for crystallization.