Abstract: A polyester product is treated with a treatment liquid containing at least one of dibromophenyl derivatives represented by the following general formula: ##STR1## wherein Y is --SO.sub.2 -- or --C(CH.sub.3).sub.2 --, each of R.sub.1 and R.sub.2 is --H, --CH.sub.3, --C.sub.2 H.sub.5, --C.sub.3 H.sub.7 OH, --CH.sub.2 CH.sub.2 Br or --(CH.sub.2 CH.sub.2 O).sub.n H and n is a positive integer of from 1 to 9 to obtain a flame retardant polyester fiber product. The polyester product may be further treated with an oligomer of vinyl phosphonate which is present either in the above treatment liquid with the dibromophenyl derivative or in a separate treatment bath from the above treatment liquid.