We studied how the photo cross-linking density of a polymeric insulator affects the mobility of organic thin-film transistors (OTFTs). The photo cross-linking density was monitored in terms of the optical anisotropy with changing the exposure time of the linearly polarized ultraviolet (LPUV) light. It was found that the increase in the mobility with the LPUV exposure time results mainly from the high packing density of pentacene molecules through the photo cross-linking of the polymer. This suggests that the packing density of pentacene molecules plays a more essential role in the enhancement of the mobility than the alignment of pentacene molecules.