The implantation of P + into GaAs has been used to compensate the loss of As during annealing of Si-implanted GaAs which affects the material quality. The implanted energy and dose range of P + were 60-180 keV and 10 12-10 15 cm -2, and those of Si + were 150 keV and (2-3) × 10 13 cm -2. Transient annealing, Polaron automatic concentration profile meter, Hall effect and photoluminescence spectrum measurements were used. The results indicate that the activation efficiency and mobility in the dually Si- and P-implanted samples are higher than those in the singly Si-implanted samples. In particular, the mobility increased by 33%. The carrier concentration profile shows that the peak concentration and active layer of the dually implanted sample is higher and thinner, respectively, and the density of Ga As is lower. Finally, the results are compared with those by other P + or As + coimplantations.