To provide the treatment of polyester fiber that particularly improves the adhesion between ethylene-propylene rubber matrix and polyethylene terephthalate fiber.

In the treatment of polyester fiber, polyethylene terephthalate fibers are treated with an agent containing polyepoxide compounds, then with another treatment agent containing resorcin-formalin-rubber latex(RFL), blocked polyisocyanate and chlorophenol. In this case, the RFL treatment agent prepared under the conditions in the specific range given in the following formulas: R/F=1/1-1/3 in molar ratio, RF/L=1/5-1/15 in weight ratio, RFL/chlorophenol = 1/1.5-1/2.5 in weight ratio, The aging time of RF: 2-4 hours, and The aging time of RFL: 16-24 hours.