Soot formation was investigated numerically with CO
2 addition in a jet-stirred/plug-flow reactor (JSR/PFR) C
2H
4/O
2/N
2 reactor (C/O ratio of 2.2) at atmospheric pressure. An updated Kazakov mechanism emphasizes the effect of the O
2/CO
2 atmosphere instead of an O
2/N
2 one in the premixed flame. The soot formation was taken into account in the JSR/PFR for C
2H
4/O
2/N
2. The effects of CO
2 addition on soot formation in different C
2H
4/O
2/CO
2/N
2 atmospheres were studied, with special emphasis on the chemical effect. The simulation shows that the endothermic reaction CO
2 + H
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CO + OH is responsible of the reduction of hydrocarbon intermediates in the CO
2 added combustion through the supplementary formation of hydroxyl radicals. The competition of CO
2 for H radical through the above forward reaction with the single most important chain branching reaction H + O
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O + OH reduces significantly the fuel burning rate. The chemical effects of CO
2 cause a significant increase in residence time and mole fractions of CO and OH, significant decreases in some intermediates (H, C
2H
2), polycyclic aromatic hydrocarbons (PAHs, C
6H
6 and C
16H
10,
etc.) and soot volume fraction. The CO
2 addition will leads to a decrease by only about 5% to 20% of the maximum mole fractions of some C
3 to C
10 hydrocarbon intermediates. The sensitivity analysis and reaction-path analysis results show that C
2H
4 reaction path and products are altered due to the CO
2 addition.