The PET/PHB copolymer materials have relatively low melt viscosity with the 60 mole％ PHB material having a value that is approximately two orders of magnitude lower than the value for PET homopolymer. The structure development during the melt spinning of liquid crystalline copolyesters with rigid backbone structure were analyzed by density, birefringence, X-ray diffraction, DSC, TMA, dynamic viscoelasticity and stress-strain measurements. As the take-up velocity increased, the birefringence of PET/PHB as-spun fibers increased. This indicated that it directly influenced initial modulus and specific stress. Non-crystalline structure of PET/PHB as-spun fiber was a lot more disordered than non-crystalline structure of PET homopolymer. As the take-up velocity increased, fibril structure of PET/PHB as-spun fiber developed and mechanical properties increased. But there is a limit to enhancing the properties of the fiber with take-up velocity, as can be seen with mechanical properties.