Numerical modelling of soot formation is conducted for an axisymmetric, laminar, coflow diffusion ethylene–air flame by two different methods to investigate the effect of flame preheating. The first method cannot account for the preheating effect, while the second one can. A detailed gas-phase reaction mechanism and complex thermal and transport properties are used. The fully coupled elliptic equations are solved. A simple two-equation soot model is used to model the soot process coupled with detailed gas-phase chemistry. The numerical results are compared with experimental data and indicate that the flame preheating effect has a significant influence on the prediction of soot yields. Both methods give reasonable flame temperature and soot volume fraction distributions. However, quantitatively the second method results in improved flame temperatures and soot volume fractions, especially in the region near the fuel inlet, although the maximum flame temperatures from both methods are slightly lower than that from the experiment. ; peer reviewed: y ; NRC Pub: Yes