A series of poly(butylene-2,6-naphthalate-co-butylene adipate) (PBNAd) copolymers were synthesized by the polycondensation method. ¹H-NMR results showed that the copolymers were random and the naphthalene/adipate ratios are close to the feeding ratios. The copolyesters showed isodimorphic cocrystallization. The degree of crystallinity decreased with increasing comonomer content. Slight variations in the interplanar spacings were observed, indicating cocrystallization. Intriguing morphologies were observed by using polarized light microscopy (PLM). β-crystals of poly(butylene-2,6-naphthalate) (PBN) and large dendrites were formed in copolymers with high BN content, especially for PBNAd 90.9/9.1. Thus, polymorphism of PBN was controlled by copolymerization. The thermodynamic analysis of cocrystallization showed limited incorporation of the minor comonomer in the crystals. The adipate units can be incorporated more easily in the PBN crystal than the bulkier naphthalate units in the poly(butylene adipate) (PBAd) crystal. The enzymatic degradation was retarded with increasing butylene naphthalate content in the copolymers. PBNAd copolymers with more than 60 mol% did not hydrolyze.