Gallium nitride (GaN)-based high electron mobility transistor (HEMT) power devices have demonstrated great potential applications due to high current density, high switching speed, and low ON-resistance in comparison to the established silicon (Si)-based semiconductor devices. These superior characteristics make GaN HEMT a promising candidate for next-generation power converters. Many of the early GaN HEMTs are devices with Schottky gate, which suffer a high gate leakage and a small gate swing. By inserting an insulator under gate metal, the MIS-HEMT is highly preferred over the Schottky-gate HEMT for high-voltage power switche, owing to the suppressed gate leakage and enlarged gate swing. However, the insertion of the gate dielectric creates an additional dielectric/(Al) GaN interface that presents some great challenges to AlGaN/GaN MIS-HEMT, such as the threshold voltage (Vth) hysteresis, current collapse and the reliability of the devices. It has been reported that the poor-quality native oxide (GaOx) is detrimental to the dielectric/(Al) GaN interface quality that accounted for the Vth instability issue in the GaN based device. Meanwhile, it has been proved that in-situ plasma pretreatment is capable of removing the surface native oxide. On the other hand, low power chemical vapor deposition (LPCVD)-Si3N4 with free of plasma-induced damage, high film quality, and high thermal stability, shows great potential applications and advantages as a choice for the GaN MIS-HEMTs gate dielectric and the passivation layer. In this work, an in-situ pre-deposition plasma nitridation process is adopted to remove the native oxide and reduce surface dangling bonds prior to LPCVD-Si3N4 deposition. The LPCVD-Si3N4/GaN/AlGaN/GaN MIS-HEMT with a high-quality LPCVD-Si3N4/GaN interface is demonstrated. The fabricated MIS-HEMT exhibits a very-low Vth hysteresis of 186 mV at VG-sweep=(-30 V, +24 V), a high breakdown voltage of 881 V, with the substrate grounded. The hysteresis of our device at a higher positive end of gate sweep voltage (VG +20 V) is the best to our knowledge. Switched off after an off-state VDS stress of 400 V, the device has a dynamic on-resistance Ron only 36% larger than the static Ron.