To enhance the field-effect mobility, we tried to control the interchain distance (or packing density of polymer chains) in polydiacetylene thin films by changing the side chain length of diacetylene monomers. We selected five different alkyl chain lengths of diacetylene monocarboxylic acid. An OFET device was prepared by a vacuum deposition of diacetylene monomer, followed by a photopolymerization under Xe-lamp excitation (bottom-contact type device). In this series of samples, we obtained a maximum field-effect mobility of 4.4 x 10(-2) cm(2)/Vs in the OFET using 10, 12-heptacosadiynoic acid (HDA). Controlling the substrate temperature from 28 to 70 degrees C, we found the optimum substrate temperature of 40 degrees C for HDA monomer deposition, and obtained a high field-effect hole mobility of 0.1 cm(2)/Vs in the bottom source-drain electrode configuration.