Under the action of mercury resonance radiation,sulfur trioxide was obtained by mercury photosensitized reaction or photochemical reaction on sulfur dioxide.Air was found suitable as the source of oxygen.The mechanism of these reactions was proposed;the values of overall quantum yields of photoreaction and photosensitized reaction at the line 1849.6 Å were found to be 1.12 and 1.39 respectively,while that of the corresponding reactions at the linf} 2537.5Å,0.31 and 0.68 respectively.