In this article,organic thin-film transistors (OTFTs) with top-gate and bottom contact geometry based on pentacene as active layer were fabricated. The experimental data of the I-V were obtained from the organic thin-film transistors. The field-effect mobility of the OTFT was calculated by fitting of theoretical calculation to the experimental data. We find that field-effect mobility values have great difference by different fitting methods. We calculate the field-effect mobility by least-squares fitting method to the experimental data of the I-V away from the center of the linear region of the transfer characteristics curves 1/2 range. The inherent inaccuracy of other fitting method can be reduced. The results is the nearest field-effect mobility obtained with other methods.