Hydrogen is the cleanest energy carrier and has been extensively used as an additive to improve the combustion properties and lower pollutant formation of other hydrocarbons. This numerical study investigated the effects of H-2 addition to fuel on the structure and soot formation characteristics of laminar coflow C2H4 diffusion flames burning in an oxidizer stream consisting of 30% O-2-70% CO2 at atmospheric pressure using the CoFlame code. Numerical calculations were conducted using a detailed gas-phase reaction mechanism and a soot model comprising the collision of two pyrene molecules as the soot nucleation step, the hydrogen abstraction acetylene addition (HACA) and pyrene condensation as the soot surface growth processes, and soot oxidation by oxygen and OH. Four H-2 addition levels of 0, 10, 30, and 50% on volumetric basis were considered. To demonstrate the chemical effects of H-2, additional calculations were performed with addition of fictitious hydrogen, FH2. The results confirm the literature finding made in C2H4/air diffusion flames that hydrogen addition to ethylene suppresses soot formation significantly through not only the dilution but also the chemical effects. The effectiveness of H-2 on soot formation suppression on per unit percentage addition basis deteriorates as more hydrogen is added to ethylene. The fundamental cause for the reduced soot formation by H-2 addition is that the added hydrogen increases the mole fraction of H-2, resulting in decreased polycyclic aromatic hydrocarbons (PAH(S)) concentrations, nucleation rates, and surface growth rates in the soot nucleation and early growth regions. The added H-2 to the fuel stream has no chemical interactions with CO2 in the oxidizer stream.