In the current work, electrical performance of n-channel GaAs MOSFETs with HfO2 gate dielectrics has been investigated by considering the impact of oxygen diffusion from gate dielectric layer. Initially, the HfO2/GaAs MOS capacitors are fabricated and its relevant process recipe has been simulated. The key parameters are extracted from both the experimental and simulated results to calibrate the simulator. The extracted parameters are subsequently fed into the simulator to investigate electrical performance of n-channel GaAs MOSFETs with varying gate lengths. The elemental diffusion of oxygen at HfO2/GaAs interface has also been incorporated since oxygen naturally diffuses into the GaAs layer during deposition and annealing steps and thereby alters the effective doping concentration in the channel. The diffused oxygen has been observed to improve electrical performance parameters such as transconductance and threshold voltage, however, degrades DIBL of the HfO2/GaAs MOSFET devices.