Au/$SnO_2$ core-shell structure nanoparticles (NPs) were synthesized by microwave hydrothermal method, and the effect of working temperature on sensitivity of Au/$SnO_2$ core-shell NPs for CO gas was investigated. The $SnO_2$ shell layer was consisted of $SnO_2$ primary particles with 4.5 nm diameter. The response of Au/$SnO_2$ core-shell NPs for CO gas was maximized at the working temperature of $350^{circ}C$ while the sensitivity increased with decreasing the working temperature due to the low grain size effect of $SnO_2$ NPs on the response of CO gas.