Organic thin film transistors (OTFTs) are expected to find the special applications, which can not be achieved by Si devices. In this paper we investigated the effects of various processes such as active layer doping, annealing, selective doping on the contact metals with self-assembly and especially surface treatment of gate insulator with organic molecules on the performance of pentacene-based OTFTs. It has been found that the selective doping on metal contacts is a good way to improve the contact property. And the usage of OTS as the organic molecule for surface treatment of gate insulator is the most effective way to enhance the field effect mobility to 0.3 cm2/V·s and the on/off ratio to 106.