We report a novel process to achieve slanted field plate (S-FP), which is a field plate with a gradual increase thickness from gate edge towards drain - utilizing grayscale lithography on flowable oxide (FOX) in single process step, in which developed FOX works as field plate dielectric. GaN-on-Si MOSHEMTs are fabricated by this technique. The breakdown voltage shows a significantly improvement by S-FP as a result of more uniform electric field distribution in the drift region. The S-FP MOSHEMT exhibits outstanding performance with a breakdown voltage (V BR ) of 832 V ( L GD = 5μm, at 1 μA/mm), state-of-the-art ON-resistance (R ON ) of 4.5 Ω·mm, and high-power figure-of-merit (FOM = V BR 2 /R ON,SP ) of 1.24 GW/cm 2 . This approach using a simple and flexible process to engineer the electric field in the MOSHEMT, offering a powerful technology for future advances in GaN high power devices.