Soot formation was investigated numerically with CO

2

addition in a jet-stirred/plug-flow reactor (JSR/PFR) C

2

H

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

2

H

4

/O

2

/N

2

. The effects of CO

2

addition on soot formation in different C

2

H

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

+ H1269CO + 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

2

1269O + OH reduces significantly the fuel burning rate. The chemical effects of CO

2

cause a significant increase in residence t

ime and mole fractions of CO and OH

significant decreases in some intermediates (H

C

2

H

2

)

polycyclic aromatic hydrocarbons (PAHs

C

6

H

6

and C

16

H

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

2

H

4

reaction path and products are altered due to the CO

2

addition.