InN thin films with different nitridation time were grown by radio frequency plasma molecular beam epitaxy (RF-MBE) on sapphire (Al₂O₃) substrate. By means of SEM and X-ray diffraction scanning, InN thin films obtained by nitridation time of less than 60 min present polycrystalline structure and rough surface morphology, while InN thin films obtained by nitridation time of 60 and 120 min have single crystal structure and less rough surface morphology. It shows that the nitridation time plays an important role in the formation of the InN crystal structure.