AlGaAs alloy was prepared using diffusion method under 650 ℃ and 800 ℃.The effect of temperature on the diffusion rate of AlGaAs alloy was investigated by Scanning Electron Microscopy(SEM). The results showed that diffusion rate decreased as the temperature increased. It was suggested that the high temperature would promote the enrichment of As atoms at the interface of Al and GaAs, which would withdraw the further diffusion of Al. The preparation time of AlGaAs under 650 ℃ was calculated in Fick law according to the experiments.