Polycrystalline hexagonal AlN thin films were prepared by DC magnetron reactive sputtering on Si(111) substrates.The influences of sputtering pressure on structure and surface roughness of the AlN thin films were investigated.The results show that:when the sputtering pressure is less than 0.6 Pa,the AlN thin films are amorphous and there is no obvious absorption in the Fourier transformation infrared spectra; when the sputtering pressure is no less than 0.6 Pa,the thin films are polycrystalline structure with the hexagonal phase (100),(110) and weak (002) peaks and an intense absorption exits at 677 cm-1 of wavenumber in the Fourier transformation infrared spectra; when the sputtering pressure increases,the surface roughness of the thin films decreases firstly then increases,while the deposition rate increases firstly then decreases,and the higher deposition rate can help to reduce the surface roughness of thin films; it has the minimum surface roughness and the maximum deposition rate when the sputtering pressure is 0.6 Pa.