Formation of soot and NO in counterflow ethylene/oxygen/nitrogen diffusion flames was numerically investigated. Detailed chemistry and complex thermal and transport properties were used. A simplified two-equation soot model was adopted. The results indicate that NO emission has negligible influence on soot formation. However, soot formation affects the emission of NO through the radiation induced thermal effect and the reaction induced chemical effect. When the oxygen index of the oxidant stream is lower, the relative influence of chemical reaction caused by soot on NO emission is more important, while the relative influence of the radiation induced thermal effect becomes more important for the flame with a higher oxygen index in the oxidant stream. ; peer reviewed: y ; NRC Pub: Yes