Pt and SnO2 were co‐functionalized on single‐walled carbon nanotubes (SWNTs) assembled on microelectrodes by electrochemical deposition where Pt nanoparticle's morphology, size, and density were tuned by controlling the applied potential and time. The systematic study to obtain the optimum condition for Pt‐decorated SnO2/SWNTs (Pt/SnO2/SWNTs) were performed and also correlate with its CO sensing performance. Illumination‐dependent sensing performance was examined using red, green and UV LED as light sources at room temperature. Under UV illumination, the sensitivity of Pt/SnO2/SWNTs was enhanced to 2.1 %/ppmV of CO with the lower detection limit of 0.05 ppmV.