$RuO_2$ thin films are prepared by RP magnetron reactive sputtering and their characteristics of crystalliBation,microstructure, surface roughness and resistivity are studied with various O2/(Ar+O2)ratios and substrate temperatures. As O2/(Ar+O2) ratio decreases and substrate temperature increases, the preferred growing plane of$RuO_2$ thin films are changed from (110) to (101) plane. With increase of the 021(Ar+O2) ratio from 2075 to 50%, the surface roughness and the resistivity of $RuO_2$ thin films increase from 2.38nm to 7.81nm, and from $103.6 muOmega-cm; to ; 227 muOmega-cm$, respectively, but the deposition rate decreases from 47nm/min to 17nm/min. On the other hand, as the substrate temperature increases from room temperature to$500^{circ}C$, resistivity decreases from $210.5 muOmega-cm; to ; 93.7muOmega-cm$. $RuO_2$ thin film deposited at $300^{circ}C$ shows a excellent surface roughness of 2.38 m. As the annealing temperature increases in the range between $400^{circ}C$ and $650^{circ}C$, the resistivity decreases because of the improvement of crystallinity. We find that RuO$_2$ thin film deposited at 20% of 02/(Ar+O2) ratio and $300^{circ}C$ of substrate temperature shows excellent combination of surface smoothness and low resistivity so that it is well qualified for bottom electrode for ferroelectric thin films.