ZnO thin films were prepared by RF magnetron sputter deposition on p-Si(100) wafer with various cooling rates of substrate temperature such as the substrates were pre-heated to $400^{circ}C$ before the deposition and then cooled down naturally or slowly to $300^{circ}C,;200^{circ}C,;100^{circ}C$, and R.T. by the temperature controller during the deposition. Crystalline and micro-structural characteristics of the films were investigated by XRD and SEM. ZnO films which cooled down naturally or slowly by the temperature controller during the deposition, especially the film were deposited with cooling down from $400^{circ}C;to;200^{circ}C$ slowly. showed the most outstanding c-axis preferred orientation.