A modifying process for polyester synthetic fiber products wherein said polyester synthetic fiber products are dipped into a treating liquor having an aggregation initiating temperature in the range of 65.degree. - 125.degree. C and then treated by heating them to temperatures at least 1.degree. C higher than said aggregation initiating temperature. Said treating liquor is prepared by adding to an aqueous dispersed liquid of a polyester polyether block copolymer (1) a anion-nonion or nonion surface active agent which serves to improve the high temperature stability of said block copolymer dispersed liquid, (2) an adsorption promoting or aggregation temperature adjusting agent which serves to increase the affinity of said block copolymer to the polyester synthetic fibers and improve the sorption power of said block copolymer and, if desired, (3) dyestuffs and carriers.