This study was focused on the maximization of flame-retardancy of polyesters by a synergism of simultaneously introduced chlorine and phosphorus into polymer chains of modified polyesters. To prepare modified polyesters, reaction intermediates, TD-adduct (prepared from trimethylolpropane /2,4-dichlorobenzoic acid (2,4-DCBA)) and TMBO (prepared from tetramethlene bis (orthophosphate)), were prepared first, then condensation polymerization of the prepared intermediates, adipic acid, and 1,4-butanediol were carried out. In the condensation polymerization, the content of phosphorus was fixed to be 2wt%, and the content of 2,4-DCBA that provides chlorine component was varied to be 10, 20, and 30wt%, and we designated the prepared modified polyesters containing chlorine and phosphorus as ABTTs. The prepared intermediates and modified polyesters were characterized with FT-IR, NMR, GPC, and TGA analysis. Average molecular weight and polydipersity index of the preparation of ABTTs were decreased with increasing 2,4-DCBA content because of the incease in hydroxyl group that retards reaction. We found that the thermal stability of the prepared ABTTs increased with chlorine content at high temperatures.