We have investigated the electrical properties of P+-implanted type IIa diamond substrates to form n-type diamond semiconductors. In this study, P+ ions were implanted at 900°C and room temperature (RT), followed by 1150°C annealing to activate the implanted P atoms. The sheet resistances and Hall effect were measured. The activation energy evaluated from the dependence of sheet resistance on the inverse absolute-temperature was estimated to be 0.6 eV from 573 K to 873 K for the P-implanted sample at 900°C. The sheet resistance of the RT-implanted samples was lower than that of the 900°C-implanted ones at all measured temperatures. However, n-type conduction was not clearly observed in the Hall effect measurements for all the samples. It is consequently suggested that primary and secondary defects formed during hot implantation and after post annealing play an important role in carrier motion in the diamond substrate.