Soot emission, known as PN (particular number), for GDI SI engines is becoming a big issue as the emission regulations being increasingly stricter. It is found that ethanol, as an oxygenated bio-fuel, can reduce the soot emission when added to gasoline. To fully understand the soot reducing effect of ethanol, the soot characteristics of ethanol/gasoline blends was studied on laminar diffusion flames. In this experiment, the blending ratio of ethanol/gasoline was set as E0/20/40/60/80. Considering the carbon content decreasing due to ethanol addition, carbon mass flow rate was remained constant. The two-dimensional distributions of soot were measured quantitatively using two-color laser induced incandescence technology. The results showed that ethanol is able to decrease the soot significantly, while the effect of ethanol on soot reduction is weakened with the ethanol ratio increasing. Compared with pure gasoline, the total soot volume fraction in the E20, E40, E60, and E80 flames decrease by 41.5%, 66.9%, 81.0%, and 93.8% respectively, and the peak soot volume fraction decrease by 34.7%, 61.7%, 74.0% and 87.8% respectively. For the whole flame, the flame lift-off height and the initial height of soot formation increase with ethanol ratio increasing. Thus, the ethanol addition can prolong the ignition delay of gasoline. In flame center line, as the flame height increasing, the soot volume fraction varies in a trend of three stages: first maintain a constant value of 0, then increase to peak gradually, and finally decrease to 0. The initial height of soot formation in flame center line also increases with the ethanol ratio increasing. The radial soot volume fraction decreases with ethanol ratio increasing at different heights above burner. As flame height increasing, the radial peak soot volume fraction moves from flame outside to flame center gradually as well as the peak soot increases first and then decreases.