We studied on possibility of the application of photolithography technique to patterning the organic active layer poly(3-hexylthiophene) (P3HT). In the case of selective etching method, we made thin oxide film on P3HT thin film using $O_2$ treatment. We achieved the field-effect mobilities in the saturation regime ${sim}1.2{ imes}10^{-3};cm^2/V{cdot}s$, $I_{on/off}$ ratios ${sim}10^5$ in the selective etching method, ${sim}7.4{ imes}10^{-4}cm^2/V{cdot}s$, $I_{on/off}$ ratios ${sim}5{ imes}10^3$ in the lift-off one. These values are higher than ones of the unpatterned P3HT-based OTFTs. On the basis of the above results, we demonstrate the photolithographic patterning for P3HT active layer is successfully carried out without degradation of P3HT.