The primary goal of this work is to investigate the capability of gate oxide scaling down in HfO2-based GaAs metal-oxide-semiconductor capacitor (MOSCAP) using a thin germanium (Ge) interfacial passivation layer (IPL). With HfO2 of 45–50 Å, an equivalent oxide thickness (EOT) of 8.7 Å was achieved with a low gate oxide leakage current density (Jg) of (2–4)×10-3 A/cm2 at VG-VFB=1.0 V. This is the thinnest EOT thickness ever reported for high-k III-V MOSCAPs. On the other hand, with thicker HfO2 of 100–110 Å, an EOT of 20–22 Å with Jg of (2–4)×10-6 A/cm2 at VG-VFB=1.0 V was attained. In addition, breakdown voltages of gate oxide and hysteresis characteristics according to different thicknesses of HfO2 were studied. The results indicate that a Ge IPL and thin HfO2 enable excellent gate oxide scaling down in GaAs system.