Gas sensor based on nano-structured tin oxide (SnO2 ) and multi-walled carbon nanotube (MWCNT) composite material has successfully been synthesized. Reflux method was used to produce SnO2-MWCNT powder with various ratio 1:0, 4:1, 2:1, 1:1 and 0:1. The XRD result shows that the synthesized material comprises of the combination of carbon elements (MWCNT) and SnO2 of the crystalite cassiterite phase. BET analysis shows that MWCNT particles increase the specific surface area of SnO2 particles. SEM images show the morphology of the SnO2 nanoparticle composite attached to the MWCNT wall with a diameter of 40-60 nm and dispersed around it. Gas sensor testing was carried out at room temperature, 50, 100, 150, 200, 250, 300, and 350o C. All sensor samples were exposed to 30 ppm CO gas for 15 minutes. . It was found that sample with 1:0 ratio gives the best response with 98.91% at 350o C. CO gas tests have also been carried out at various concentration 10, 30, 50 and 70 ppm to 4:1 SnO2-MWCNT sample at 150o C. The higher the CO gas concentration, the greater the response. SnO2-MWCNT with 4:1 ratio at 50o C has the fastest response time of 10 s and the fastest recovery time of 3 s.