Changes in structural characteristics and band alignments of atomic-layer-deposited HfO2 films on InP (001) as a function of annealing temperature and film thickness were investigated using various analytical techniques. After an annealing at temperatures over 500 °C, the HfO2 films were converted into a fully crystalline structure with a tetragonal phase with no detectable interfacial layer between the film and the InP substrate. In–P–O states, produced by interfacial reactions, were increased during the post deposition annealing (PDA) process and oxides were detected in the surface region of the HfO2 film, indicating that In and P atoms had out-diffused. The Eg value of the as-grown HfO2 film was found to be 5.80 ± 0.1 eV. After the PDA treatment, the optical band gap and valence band offset values were significantly affected by the interfacial oxide states between the HfO2 film and InP substrate. Moreover, band bending in InP, due to negative space charges generated by an unstable P-rich interfacial state during atomic layer deposition process was decreased after the annealing treatment.