We present the underlying mechanism for the mobility enhancement in a pentacene thin-film transistor TFT with a photocrosslinking polymeric insulator, polyvinyl cinnamate PVCi. Experimental results for the optical anisotropy, x-ray diffraction, and microscopic layer-by-layer coverage of the pentacene film on the photocrosslinked PVCi layer, exposed to a linearly polarized ultraviolet LPUV light, clearly demonstrate the importance of the structural packing of pentacene grains rather than the directional alignment of the pentacene molecules for the mobility enhancement. The packing density of pentacene grains is directly related to the number of photocrosslinking sites of the PVCi insulator produced by the LPUV. It is found that the mobility in the pentacene TFT is linearly proportional to the number of photocrosslinked sites of the PVCi insulator serving as interaction sites for the layer-by-layer coverage of the pentacene molecules with no preferred orientation. © 2007 American Institute of Physics.