The authors fabricated OTFT devices with different insulator materials. The OTFTs with PMMA as the dielectric layer exhibit better properties, including a mobility of 0.207 cm2·V-1·s-1, an on/off current ratio of 4.93×103, and a threshold voltage of 4.3 V. However, the OTFTs based on oxidized silicon dielectric layer perform not so well, with a mobility of 0.039 cm2·V-1·s-1, an on/off current ratio of 5.98×102, and a threshold voltage of 5.4 V. In order to explain the difference in performances, we compared the surface roughness of the oxidized silicon film with that of the PMMA film according to the results of atomic force microscopy, and found that the former had a roughness mean square (RMS) of 1.579 nm and the latter was more smooth with an RMS of 0.216 nm. The quality of the pentacene films deposited onto oxidized silicon and PMMA was also studied by atomic force microscopy and X-ray diffraction. From the results of AFM, the authors found that the pentacene film deposited on PMMA had high thin film quality with larger grain size and less crystal grain boundaries. From the results of XRD, the authors found that the pentacene film deposited on PMMA had clear diffraction peak, showing that the pentacene film deposited on PMMA had greater crystallite quality once again. Therefore, OTFTs with PMMA as insulator layers have advantages over those OTFTs with oxidized silicon dielectric layer.