The effect of anions, PF6 − and ClO4 −, on the oxidation decomposition mechanism of electrolyte solvent ethylene carbonate (EC) for lithium ion battery has been investigated by using density functional theory at B3LYP/6-311++G(d) level, in both gas phase (dielectric constant ɛ = 1) and solvent (dielectric constant ɛ = 20.5). It is found that the presence of PF6 − and ClO4 − anions significantly reduces EC oxidation stability and changes the order of the oxidation decomposition paths. The primary oxidation decomposition products of EC, EC–PF6 − and EC–ClO4 − are CO2 and acetaldehyde radical. The formation of CO is highly improbable in the decomposition reaction of EC with and without PF6 − or ClO4 −.