The device performances of pentacene-based organic thin film transistors (OTFTs) were greatly improved by irradiation of infrared light. The field effect mobility and maximum drain current increase from 0.20±0.01 to 0.57±0.02 cm2/V s and 1.14×10−5 to 4.91×10−5 A, respectively. The (001) peak of the pentacene “thin film” phase increases in intensity by 4.5 times after infrared irradiation at 50 W for 2 h. Two types of crystal orientations, i.e., “crystal I” (2θ=5.91°) and “crystal II” (2θ=5.84°), coexist in the pentacene. The improvement of the characteristics of OTFTs is attributed to crystallization and crystal reorientation induced by infrared light.