Alcohols are widely used as alternative fuel for spark ignition engines to reduce soot emission. 2D soot distributions of alcohols-gasoline blends were studied in laminar diffusion flames by two-color laser-induced incandescence technique. The soot volume fraction decreases significantly with the alcohol blending ratio. At the same blending ratio, the soot-reducing ability declined with the carbon length of the alcohol, but the reducing effect for short-chain alcohols, i.e., methanol and ethanol, deteriorated at high blending ratio while it remained constant for the long-chain alcohol n-butanol. At fixed oxygen content, the soot-reducing ability of a short-chain alcohol is still higher than that of a long-chain alcohol, i.e., not only the oxygen content but also the molecular structure dominates the soot-reducing ability.