The optical and photoelectric properties of pentacene films deposited on n-Si by thermal evaporation at room temperature, 60 and 80 degreesC are investigated. Ellipsometric spectra of the films deposited at room temperature and 60 degreesC exhibited a main absorption peak at 1.82 eV and additional weak features at higher energy levels. Photoelectric effects were well observed from p-pentacene/ n-Si diodes, which showed a maximum photoresponsivity of 3.87 A/W under a monochromatic red light of 1.85 eV (670 nm) at 5 V reverse bias. All the diodes exhibited typical rectifying behavior but a higher dark current leakage level was observed from the p-pentacene/n-Si diodes prepared at a higher deposition temperature.