Pentacene thin films were prepared by vacuum evaporation on SiO2.Si (100) and glass substrates with various growth conditions. X-ray diffraction, atomic force microscopy, and electrical conductivity measurements were employed to characterize the various properties of the films. Two distinct crystalline phases were observed with the 001 spacing of 15.0Angstrom (thin film phase) and 14.0Angstrom (single crystal phase), respectively. The single crystal phase appeared over a critical thickness of the thin film phase, and the critical thickness was strongly dependent on growth conditions: increasing the substrate temperature decreased the critical thickness. The substrate temperature enhanced the electrical conductivity of the films parallel to the substrate surface, although the substrate temperature induces the single crystal phase. On the other hand, gaps generated between grains degraded conductivity at a too high substrate temperature. The maximum conductivity of 2.0 x 10(-7)S/cm was gained at a substrate temperature of 40degreesC.