A mild strategy to form ethylene glycol via photocatalytic C-C coupling of methanol and formaldehyde was developed with high atomic efficiency over M/TiO2 under UV-light irradiation. The presence of both methanol and formaldehyde significantly enhanced the formation of ethylene glycol with selectivity of 80%. A radical reaction mechanism for the photocatalytic C-C coupling of methanol and formaldehyde into ethylene glycol was proposed based on electron spin resonance and isotope labeling experiment as well as quantum chemical calculations. [GRAPHICS] .