In/Pd-doped SnO₂ is synthesized via a sol–gel method and coated on a silicon substrate with Pt electrodes to fabricate a microstructure sensor. The sensor shows high response to CO with very low cross response to common interference gases at an operating temperature of 140 °C. Especially, the sensor can detect CO down to 1 ppm (the response value is about 3), and the response time and recovery time are about 15 and 20 s, respectively. These results make our sensor a good candidate in practical CO sensors.