Nylon 66 copolymers were prepared by substitution up to 40 mol% of diacid comonomers: 2, 5-dichloroterephthalic acid (TCl2) and 2, 3, 5, 6-tetrachloroterephthalic acid (TCl4). The thermal properties measured by DTA were compared with those of previously reported nylon 66 copolymers with terephthalic acid (T) and isophthalic acid (I).ηsp/C decreases with increasing comonomer content. Melting point depression shows that copolymers with TCl2 and TCl4 are non-isomorphous. Glass transition temperature (Tg) increases in the order of copolymers with TCl4 TCl4>TCl2>I. Solubility increases by the copolymerization. Flame-retardancy estimated by TG and self-ignition test increases with increasing chlorine content of copolymers.