The effective work function (Φm,eff) of TaN on HfO2 after postmetallization annealing (PMA) was investigated using TaN/HfO2/SiO2/Si as a sample structure. We found that Φm,eff on HfO2 is stable at PMA temperatures of less than 600 °C and is 4.6 eV, which is approximately 0.2 eV higher than that on SiO2. In contrast, Φm,eff is modulated by PMA at temperatures greater than 750 °C. An analysis by x-ray photoelectron spectroscopy suggests that the increased Φm,eff is strongly related to Ta oxide formation near the TaN/HfO2 interface. The modulation of Φm,eff on HfO2 is discussed on the basis of intrinsic and extrinsic Fermi level pinning due to Ta–O bond formation at the TaN/HfO2 interface.