Abstract: An organic phototransistor was fabricated by an innovative spray technology suited to large-area, low-cost and room temperature processing for organic electronic and thin-film devices. Top contact organic thin film transistors (OTFTs) were fabricated with poly(3-hexylthiophene) (P3HT) active layer and low cost polymerthylmethacrylate (PMMA) insulator，which exhibit a good output and the transfer characteristics of P3HT transistors. The OTFT device based on P3HT was measured under a standard solar simulator circumstance of 100 mW/cm2. The responses of the OTFT are remarkable in the absence or the presence of light, with good reversibility and repeatability. It was found that the threshold voltage shift was obvious, after comparing the characteristic curves of the OTFT exposed to darkness and light for 1 min, 2 min, and 4 min.