We have fabricated the top-contact organic thin film transistors （OTFTs） with, a 60 nm thick pentacene films as active layer and a 120 nm thermal growth SiO2 as gate insulator. Through using different self-assembled monolayers ,such as octadecyltrichlorosilane （OTS） and phenyltrimethoxysilane （PhTMS）, as a buffer between the organic semiconductor active layer and gate insulator, we studied the effect of different buffers on the performance of OTFTs device. At the same time, we also investigated the field-effect behavior and carrier transport mechanism of OTFTs device with PhTMS buffer under low gate modulation voltage. When |VGS|IDS of OTFTs device keeping balance. But the OTFT device still has good output characteristics, with the field-effect mobility （μEF） of 3.22×10-3 cm2/Vs, （on/off） current ratio of 1.43×102, and threshold voltage （VTH） of -0.66 V.