Thermal diffusion, caused by temperature gradients, tends to draw lighter molecules to warmer regions and to drive heavier molecules to cooler regions of a mixture. The influence of thermal diffusion on soot formation in coflow laminar ethylene/air diffusion flames is numerically investigated in this paper. Detailed reaction mechanisms and complex thermal and transport properties are employed. The fully elliptic governing equations are solved. Radiation heat transfer from the flames is calculated by the discrete-ordinates method coupled to an SNBCK-based wide band model. A simplified two-equation soot model is used. The interactions between soot and gas-phase chemistry are taken into account. The thermal diffusion velocities are calculated according to the thermal diffusion coefficients evaluated based on multicomponent properties. The results show that thermal diffusion does affect soot formation in ethylene/air diffusion flames. Although the effect on soot formation in pure ethylene/air flame is not significant, the influence is enhanced if lighter species, such as helium, are added to the fuel or the air stream. The peak integrated soot volume fraction doubles if thermal diffusion is not taken into account in the simulation of the flame with helium addition to the air stream. ; peer reviewed: ; NRC Pub: Yes