Sooting characteristics in laminar diffusion flames have been studied experimentally for ethylene-propane mixtures, and a synergistic effect from mixture fuels has been observed. Soot volume fraction and polycyclic aromatic hydrocarbon (PAH) concentration are increased when a small amount of propane is added to ethylene diffusion flames. Considering C2H2 concentrations and adiabatic flame temperatures, the experimental results indicate that H-abstraction-C2H2-addition (HACA) reaction alone is insufficient to explain soot inception and growth processes in diffusion flames of ethylene-propane mixtures. The synergistic effect of fuel mixture is discussed, emphasizing the importance of C-3 recombination reaction during the initial ring formation process for incipient PAHs. The results of soot number density and particle size in low-temperature soot growth region also support continuous soot inception via PAH agglomeration. Therefore, it is suggested that PAHs are directly involved in the soot particle inception processes. The synergistic effect is less significant for ethylene-ethane mixtures compared to ethylene-propane mixtures.