Temperature effects on interaction between P and GaAs (100) surface have been studied by in-situ X-ray photoemssion, uhra violet photcemission, high resolution electron energy loss specrroscopy and low energy electron diffraction techniques. The results show that annealing will make most of the amorphous P, which are deposited on GaAs(lOO) surface at room temperature, desorb with some randomly distributed P-clusters left on the surface. In these clusters, some of the P atoms are bonded to the Ga atoms of the substrate, others exist in the form of element. Higher temperature annealing will make all the left P atoms interact with the substrate and form GaAsP thin layer. Film of GaAsP solid solution will be obtained when depositing P on high temperature GaAs substrate. This film is suggested to be a promissing passi-vating film for GaAs surface.