In this study, we investigated microstructure and the CO gas sensing properties of Ag-CuO-$SnO_2$ thin films prepared by co-evaporation and subsequently thermal oxidation at air atmosphere. The sensitivity of a Cu-Sn films, thermally oxidized at $600^{circ}C$, is strongly affected by the amount of Cu. At Cu:7 wt%-Sn:93 wt%, the film exhibited a maximum sensitivity of ${sim}2.3$ to CO gas of 1000 ppm at $300^{circ}C$. In contrast, the sensitivity of a Sn-Ag film did not change significantly with the amount of Ag. An enhanced sensitivity of ${sim}3.7$ was observed in the film with a composition of Ag:3 wt%-Cu:4 wt%-Sn:93 wt%, when thermally oxidized at $600^{circ}C$. In addition, this thin film shows a response time of ${sim}80$ sec and a recovery time of ${sim}450$ sec to 1000 ppm CO gas. The results demonstrate that the CO sensitivity of the Ag-CuO-$SnO_2$ thin films may be closely associated with coexistence of $SnO_2$ and SnO phase, decrease in average particle size, and a porous microstructure. We also suggest that co-evaporation and followed by thermal oxidation is a very simple and effective method to prepare oxide gas sensor thin films.