The effects of post-annealing of high-k $HfO_2$ thin films grown by atomic layer deposition method were investigated by the annealing treatments of $400-600^{circ}C$. $Pt/HfO_2/p-Si;MOS$ capacitor structures were fabricated, and then the capacitance-voltage and current-voltage characteristics were measured to analyze the electrical characteristics of dielectric layers. The X-ray diffraction analyses revealed that the $500^{circ}C-annealed;HfO_2$ film remained to be amorphous, and the $600^{circ}C-annealed;HfO_2$ film was crystallized. The annealing treatment at $500^{circ}C$ resulted in the highest capacitance and the lowest leakage current due to the reduction of defects in the $HfO_2$ films and non-crystallization. Our results suggest that post-annealing treatments are a critical factor in improving the characteristics of gate dielectric layer.