The involvement of hydrocarbons such as C2H4 and its combustion intermediate species C2H2 in thermal radi- ation has not been accounted in the numerical simulations of literature studies, which may in turn cause errors in estimating the soot formation processes. Numerical calculations were conducted using detailed gas-phase chemistry and thermal and transport properties in laminar coflow ethylene/air diffusion flames. The SNBCK model parameters for C2H2 and C2H4 were generated based on HITRAN database. The results show that the position of soot formation is affected by the radiation absorption of C2H4 at low temperatures and the radiation emission of C2H2 at high temperatures. The maximum C2H2/C2H4 radiation effect is 9.46% for air condition case and 9.87% for oxygen-enriched case. The height corresponding to the maximum soot volumetric fraction in- creases for the air condition while it decreases for the oxygen-enriched condition when the radiation effect is considered. The calculations reproduced well the experimental data of soot volumetric fraction in the literature and the numerical results were improved by 10.4% when considering the C2H2/C2H4 radiation. The results indicate that the radiation heat transfer of C2H2 and C2H4 needs to be taken into account in the numerical modeling of the ethylene/air diffusion flames.