Carbon dioxide (CO2) is an important compound for the inhibition of combustion flame. This work investigated the effects of CO2 addition on the soot particle sizes in axisymmetric laminar co-flow ethylene/air diffusion flames, as determined by using temporal laser-induced incandescence (LII). Additionally, the flame height and absorption function of soot particles were also studied. The results showed that the size and absorbency of soot particles decreases with the addition of CO2. The total flame height also decreases with CO2 addition, while the height of the dark region grows. These results show that CO2 addition dilutes the gas stream, compresses the nucleation of soot particles and makes them less mature. This study provides useful information for the control of soot particle formation during combustion processes.