0.5wt% Ru/$alpha-Al_2O_3$ catalysts are prepared by deposition-precipitation method for the preferential CO oxidation In order to investigate the effect of pH on the Ru dispersion and particle size, the pH of precursor solution is adjusted to between 5.5 and 9.5. 0.5wt% Ru/$alpha-Al_2O_3$ catalyst prepared at the pH of 6.5 has high Ru dispersion of 17.9% and small particle size of 7.7nm. In addition, 0.5wt% Ru/$alpha-Al_2O_3$ catalyst prepared at the pH 6.5 is easily reduced at low temperatures below $150^{circ}C$ due to high dispersion of $RuO_2$ particle and shows high CO conversion over 90% in the wide temperature range between $100^{circ}C$ and $160^{circ}C$. Moreover, the deposition-precipitation is a feasible method to improve the Ru dispersion as compared to the impregnation method. The 0.5wt% Ru/$alpha-Al_2O_3$ catalyst prepared by deposition-precipitation exhibits higher CO conversion than 0.5wt% Ru/$alpha-Al_2O_3$ catalysts prepared by impregnation due to higher metal dispersion and better reducibility at low temperature.