A novel surface treatment of an SnO2-based sensor element using a thiourea solution has been developed. The surface treatment is accomplished by dipping the sensor element in the dilute thiourea solution for only a few seconds followed by calcination. The sensor element modified by this simple procedure shows remarkable improvements in the sensitivity and selectivity for CO detection over a certain thiourea concentration range. The long-term transients of these gas-sensing properties were also demonstrated for over 450 days. The results prove that the performance of the SnO2-bassd CO gas sensor is drastically stabilized by this surface modification, thus promising significant enhancement in the stability and reliability for CO gas alarm applications.