Two kinds of semi-aromatic polyamide terpolymers based on the isomorphic replacement copolymer system of poly(hexamethylene adipamide-co-hexamethylene terephthalamide) (PA 66/6T), that is, poly(hexamethylene adipamide-co-hexamethylene terephthalamide-co-hexamethylene sebacamide) (PA 66/6T/610) and poly(hexamethylene adipamide-co-hexamethylene terephthalamide-co-hexamethylene isophthalamide) (PA 66/6T/6I), were synthesized by melt polycondensation in order to study the effect of comonomer structure on the properties of terpolyamides. Although the incorporation of the 610 or 6I in the PA terpolymers led to overall depression in the thermal properties of terpolymers, PA 66/6T/610 exhibited noticeably higher thermal properties such as melting point ($T_m$), heat of fusion (${Delta}H_m$), and tensile mechanical performances as well as lower water absorption than those of PA 66/6T/6I. The lower water absorption might be originated from the enhanced chain mobility and the lower density of amide bond owing to the long aliphatic chain of the sebacic acid. On the other hand, the glass transition temperatures ($T_g$) of PA 66/6T/6I copolymers increased with the increment of the 6I content, which was resulted from the reduced chain mobility due to the rigid aromatic structure of the comonomer.