Triphenyl phosphate (TPP) and its analogs are known to be the most effective flame retardant for acrylonitrile-butadiene-styrene copolymer (ABS) among various phosphorus-based compounds. But, its evaporation temperature is quite lower than the processing temperature of ABS. Therefore, it is inevitable to avoid a considerable amount of TPP to evaporate during processing, In order to overcome this undesirable phenomena, we incorporated various epoxy resins to TPP as coflame retardants and a series of ABS/TPP/epoxy compounds were made from them and their flame retardancy were evaluated by measuring the limiting oxygen index (LOI) values. Our results showed that the incorporated epoxy is very effective in suppressing the evaporation of TPP from the compounds and the LOI value as high as 38 is obtained. It is also found that more the epoxide ring contents in epoxy resins, the higher the LOI value of the compounds. The reason for this finding was postulated to come from the interaction between phosphoric acid and carboxylic acid generated from epoxy during thermal degradation. And some direct evidences from FTIR experiments were presented even though they were not conclusive. Moreover, the LOI values of quaternary mixtures of ABS, tetra-2,6-dimethyl phenyl resorcinol diphosphate, TPP, and epoxy resins were evaluated and that the same synergism of epoxy incorporation on flame retardancy was also found for them.