Phosphorus ions are implanted into nanocrystalline diamond (NCD) films followed by being annealed at different temperatures. The results show that the samples exhibit good n-type conductivity when annealing temperature is increased to 800 ℃ and above. Raman spectroscopy and electron paramagnetic resonance measurements display that the sample with a larger quantity of diamond phase with better lattice perfection has a lower resistivity. It is indicated that nano-sized diamond grains make contributions to the n-type conductivity in the films. After 1000 ℃ annealing, the amorphous carbon grain boundaries become more ordered, which leads the dangling carbon bonds to decrease and the resistivity of the film to increases. It is revealed that the amorphous carbon grain boundaries supply a conduction path to the n-type phosphorus ion implanted nanocrystalline diamond grains.