A new proposed gas sensor with stack electrodes was built to archive an ultra small gap of electrodes by using the metal layers in the standard CMOS process. Beyond the conventional finger-type electrodes, the stack electrodes formed from the inter metal layers of CMOS process provide a lower resistance contact with the sensing material. The proposed gas sensor was built with array of floating membranes and serially connected over an etched cavity. A micro heater of n-type polysilicon with 5.81 kΩ is situated beneath each active area of the membrane to obtain a stable working temperature with several power consumptions conditions. The sensing material SnO 2 for CO detection was deposited onto the stack electrodes after sol-gel formation with deionized water and ammonia (NH3) to promote the hydrolysis and condensation. After the sensor fabricated, a test wire-bond and TO-5 package was performed and the sample was test and verified inside a CO gas chamber. With careful investigation of measurement results, the sensitivity of proposed monoxide sensor is 6.25%/ppm without heating and the sensitivity reaches to 15.7%/ppm under 172 μW heating power which is highly beyond the result of previous research work. The experimental measurement shows the research is applicable for a low cost CO sensor with high sensitivity.