In order to investigate the effect of fuel mixing on PAH and soot formation, four species of methane, ethane, propane and propene have been mixed in counterlfow ethylene diffusion flame. Laser-induced incandescene and laser-induced fluorescene techniques were employed to measure soot volume fraction and polycyclic aromatic hydrocarbon (PAH) concentration, respectively. Results showed that the mixing of ethane (or propane) in ethylene diffusion flame produces more PAHs and soot than those of propene. Considering that propene directly dehydrogenates to propargyl radical, this behavior implied that the enhancement of PAH and soot formation by the fuel mixing of ethylene and ethane (or propane) cannot be explained solely by propargyl radical directly dehydrogenated from ethane (or propane). Thus, combination reactions between C1 and C2-species for the formation of propargyl was suggested to identify the synergistic effect occurring in the flames of ethylene and propane (or ethane) mixtures.