By optimizing the technique and conditions experimentally, we have grown GaN film on vicinal sapphire (0001) substrates by radio frequency plasma-assisted molecular beam epitaxy (MBE). It was found that the films grown on vicinal sapphire (001) substrates have better quality than that grown on conventional substrate, as shovn by XRD and AFM characterization. Through investigation of the instantaneous relaxation behaviors of photoconductivity in GaN films grown on vicinal and common sapphire substrates, three stages of carrier recombination in the conventional MBE GaN film were discovered. The stages consist of bimolecular, monomolecular and persistent recombination phases in which the relaxation times are 0.91, 7.7 and 35.5ms, respectively. In comparison, only bimolecular and monomolecular recombination processes of photo-generated carriers were found in the GaN film grown on vicinal sapphire (0001) substrate, the relaxation time was measured to be 0.78 and 14 ms, respectively. Theoretical considerations directly show that the persistent conductivity mainly originates from the native dislocation defects in GaN film grown by MBE.