Hydroxy-terminated poly(pentamethylene 2,6-naphthalenedicarboxylate) oligomers were prepared by melt polycondensation of dimethyl 2,6-naphthalenedicarboxylate with excess 1,5-pentanediol followed by evacuating out some 1,5-pentanediol. The molecular weight of the poly(pentamethylene 2,6-naphthalenedicarboxylate) oligomers was controlled by the charge molar ratio of 1,5-pentanediol to dimethyl 2,6-naphthalenedicarboxylate and the amount of 1,5-pentanediol removed under vacuum. The H-1-NMR spectra of the poly(pentamethylene 2,6-naphthalenedicarboxylate) oligomers indicate that the transesterification between dimethyl 2,6-naphthalenedicarboxylate and 1,5-pentanediol was almost complete. Block copolyesters with hard segments of poly(pentamethylene 2,6-naphthalenedicarboxylate) and soft segments of poly(tetramethylene adipate) were prepared by coupling the poly(pentamethylene 2,6-naphthalenedicarboxylate) oligomer and a poly(tetramethylene adipate) glycol with methylene-4,4'-diphenylene diisocyanate in solution. The block copolyesters were characterized by IR, H-1-NMR, DSC, and X-ray diffraction. The hard segments in the block copolyesters display an amorphous state. However, the thermal transitions of soft segments in the block copolyesters are strongly dependent on the composition. When the content of the hard segments increases, the glass transition temperature of the soft segments increases. Thus, the amorphous parts of the soft segments would be partially miscible with the hard segments. When the content of the hard segments is very low, the soft segments of the block copolyesters exhibit high crystallinity. But, as the content of the hard segments is about 30 wt % or more, the soft segments of the block copolyesters become amorphous. This is described as the effect of the presence of the hard segments which are partially miscible with the soft segments.