We have investigated the gate insulator effects on the electrical performance of p-type tin monoxide (SnO) thin-film transistors (TFTs). Various SnO TFTs are fabricated with different gate insulators of a thermal $SiO_2$, a plasma-enhanced chemical vapor deposition (PECVD) $SiO_x$, a $150^{circ}C$-deposited PEVCD $SiO_x$, and a $300^{circ}C$-deposited PECVD $SiO_x$. Among the devices, the one with the $150^{circ}C$-deposited PEVCD $SiO_x$ exhibits the best electrical performance including a high field-effect mobility ($=4.86cm^2/Vs$), a small subthreshold swing (=0.7 V/decade), and a turn-on voltage around 0 (V). Based on the X-ray diffraction data and the localized-trap-states model, the reduced carrier concentration and the increased carrier mobility due to the small grain size of the SnO thin-film are considered as possible mechanisms, resulting in its high electrical performance.