Pentacene is a small molecule organic semiconductor and has high absorption in the UV and visible region. In this work, we report the channel-length-dependent performance of bottom-gate top-contact photosensitive organic field-effect transistors based on pentacene with different channel lengths ranging from 25 to 150 μm. The results show that, for a given drain voltage, the photoresponsivity, external quantum efficiency, and specific detectivity decrease with increasing channel length. For a given channel length and gate voltage, the photoresponsivity, external quantum efficiency, and specific detectivity increase with drain voltages, and then tend to saturate at high drain voltages.