The mechanical properties of fiber molded samples and monofilaments of thermally treated 73/27 4-hydroxy benzoic acid/2-hydroxy-6-napthoic acid (HBA/HNA) copolyester have been investigated using both tensile tests and flexural three-point bending tests. The thermal treatment which involves step annealing at temperatures well below the degradation temperature of the 73/27 system has been shown to produce branching and crosslinking in the crystalline regions of these polymers. The flexural strength of the degraded sample decreased up to 10% of the untreated fiber molded sample. In case of tensile strength of a single fiber, the values for the degraded samples are in line with the untreated fiber in the low draw ratio region while a slight decrease in tensile strength was observed in the high draw ratio region. The decrease in flexural and tensile strength appears to result from a small amount of branching and crosslinking reactions which arise uniquely in the orthorhombic phase of the 73/27 HBA/HNA copolyester. The branching and crosslinking would prevent the molecular orientation along flow direction in the molten state. For the fiber molded samples of degraded 73/27 HBA/HNA the destruction of the chain regularity along fiber axis direction was observed by wide-angle X-ray diffraction. The 73/27 HBA/HNA copolyester including 1 wt% of a crosslinked oligomer was used to simulate the branching and crosslinking of the degraded 73/27 HBA/HNA copolyester. Plots of tensile strength versus draw ratio were similar for the degraded 73/27 HBA/HNA and a copolyester which included 1 wt% of a crosslinkable oligomer.