The growth mechanism and crystallization phase state were investigated by the methods of atomic force microscopy (AFM) and X-ray diffraction (XRD). The pentacene films were deposited with a self-assembling monolayer by thermal evaporation on p+-Si wafer substrates at room temperature and annealed at a constant temperature (80 ℃) for 120 min. The experimental results show that pentacene films were grown with terraces island structure with the diameter of island of about 100 nm and constituted a layer consisting of faceted grains with a average step height between terraces of 1.54 nm·s-1, which were accord with the long axis length of pentacene molecule, and the film were vertically grown on the substrate surface. The crystallization of pentacene thin films is shown in XRD pattern. The increase in the thin film thickness introduced a second set of diffraction peaks, which were attributed to the pentacene triclinic bulk phase. The critical thickness of both phases is 150 and 80 nm, respectively. At a film thickness of 150 nm, the triclinic phase diffraction peaks become the dominant phase. This is contrast to the XRD spectrum of very thin film of 80 nm thickness, where the thin film phase is the only contribution.