Eco-friendly flame retardant poly(butylene terephthalate) copolymers (FR-PBTs) were synthesized with various amounts (5.5, 8, 10 and 15%) of the flame retardant, phosphorous comonomer unit, [(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)methyl]-methyl butanedioate (DOP-MBDA). Thermal stability was determined by TGA and the resistance to hydrolysis was examined by comparing the inherent viscosities of the polymers before and after hydrolysis in a 10 wt% aqueous NaOH solution at $130^{circ}C$ for 8 h. Unfortunately the thermal stability and hydrolytic resistance of the copolymer decreased gradually with increasing DOP-MBDA content up to 10%. However, when the DOP-MBDA content was increased from 10 to 15% (FR-PBT-15), the thermal stability and hydrolytic resistance improved dramatically to those of the PBT homopolymer. FTIR spectroscopy showed that the ratio of the trans to gauche moiety in the copolymer chain decreased with increasing DOP-MBDA content up to 10%, whereas the ratio increased when 15% DOP-MBDA was incorporated into the polymer. The higher trans to gauche ratio suggests that crystallization may take place more readily in FR-PBT-15. In addition, FR-PBT-15 showed the highest density and degree of crystallinity among the copolymers. The improved thermal stability and resistance to hydrolysis of FR-PBT-15 was attributed to the denser stereo-stable crystal structure of the polymer.