The purposes of this paper are to investigate the post deposition annealing (PDA) effect on structural and electrical characterizations of HfO2 MOS capacitor and the frequency dependency of series resistance and interface states in this device. PDA processes on the HfO2 films deposited using RF magnetron sputtering system were performed in N-2 ambient at 350, 550, 650, and 750 A degrees C. The phase identifications and crystallization degrees of the HfO2 films were determined by using X-ray diffractometry. The grain size of the films was varied from 4.5 to 15.23 with increasing in PDA temperature. The HfO2 MOS capacitors were fabricated using the as-deposited and annealed films for electrical characterization. C-V and G/omega-V measurements were performed at 1 MHz frequency. The C-V characteristics of the MOS capacitor fabricated with film annealed at 550 A degrees C show a better behaviour in terms of the high dielectric constant and low effective oxide charge compared to others. For this device, C-V and G/omega-V measurements were performed in different frequencies ranging from 10 kHz to 1 MHz at room temperature. Obtained results show that series resistance and interface states strongly influence the C-V and G/omega-V behaviour of the MOS capacitor.