Ultrathin HfO2 gate dielectrics with 3.00 and 2.60 nm thicknesses were prepared at 300 °C by plasma-enhanced metal-organic chemical-vapor deposition. These HfO2 films were treated by N2 plasma at 300 °C, and exhibit not only an excellent smooth surface morphology but also higher crystallization temperature and improved electrical properties. The N2-plasma treatment did not influence the interface charge density of the films. The 2.6 nm HfO2 treated by N2 plasma showed a leakage current density of 3×10-3 A/cm2 at -1.5 V, equivalent oxide thickness of 1.24 nm, and flatband voltage of 2 mV. We find that N2-plasma treatment in Hf-based ultrathin gate dielectrics can improve the electrical properties, as well as thermal stability.