As far as the flame-retardant polyester fibers are concerned, the copolymerization of phosphorus retardants is the most common method. But a serious problem is that the phosphorus-containing polymer is easily hydrolyzed. We investigated the flame retardancy and the hydrolysis properties of two poly(ethylene terephthalate) (PET) fibers, one with a phosphorus compound as a side chain (side-chain type: HEIM(R) Toyobo Co., Ltd.), and one with a phosphorus compound inserted in the polymer backbone (main-chain type). Both types had almost the same properties of fibers and flame retardancy, but the main-chain type was hydrolyzed about two times faster than the side-chain type, and led to a decrease of toughness immediately. This difference of hydrolysis properties between main-chain type and side-chain type depends on whether a phosphonate ester bond is placed in the polymer backbone or the pendant site. In the case of the main-chain type, the scission of the polymer backbone chain occurs by hydrolysis of phosphonate ester bonds; however, in the case of the side-chain type, this does not occur. These results demonstrate that the flame-retardant polyester fiber with the side-chain type modifier gives sufficient flame retardancy and excellent hydrolysis resistance.