Pentacene-based organic field-effect transistors (OFETs) with SiO₂ and HfON gate insulators have been fabricated, and the effect of gate insulator on the electrical properties of pentacene-based OFETs and the microstructures of pentacene films were investigated. It was found that the grain size for pentacene film deposited on HfON gate insulator is larger than that for pentacene film deposited on SiO₂ gate insulator. Due to the larger grain size, pentacene-based OFET with HfON gate insulator shows better electrical properties compared to pentacene-based OFET with SiO₂ gate insulator. Meanwhile, low-temperature (such as 140°C) fabricated pentacene-based OFET with HfON gate insulator was also investigated. The OFET fabricated at 140°C shows a small subthreshold swing of 0.14V/decade, a large on/off current ratio of 4×10⁴, a threshold voltage of -0.65V, and a hole mobility of 0.33cm²/Vs at an operating voltage of -2V.