We studied the carrier profile tail formed in GaAs materials of 28Si+ implanting into Cr-doped semi-insulating substrate, including the effects of annealing temperature and time. The effects of carrier profile tail on GaAs MESFET output characteristics simulated by computer were in accord with measured results, indicating that the profile tail does make the abnormal output characteristics in MESFET. The origin of profile tail in connection with Si background in GaAs substrate and Cr redistribution during high temperature annealing was discussed.