Organic thin film transistors with P3HT (poly-3-hexylthiophene) as active semiconducting layer, channel lengths from 0.3 to 20 mu m, and gate oxide thicknesses from 15 to 170 nm have been successfully fabricated on Si substrates. The measurement results show that the channel length over oxide thickness ratio should be large enough (i.e., the vertical electric field should be at least 10 times higher than the lateral electric filed) in order to suppress the short channel effects of transistors. The field effect mobility of long channel devices (L a parts per thousand yen 5 mu m) is about an order of magnitude larger than small channel devices (L from 0.3 to 2.5 mu m), which could be attributed to the more severe contact resistance effects between organic materials and metal contacts for devices with smaller dimensions.