Biodegradable poly(butylene terephthalate/succinate/adipate) (PBTSA) pellet, an ideal random copolymer characterized by H-1 solution NMR, was melt-spun into fibers. The crystal structure and physical properties of the as-spun fibers were investigated by WAXD, solid-state C-13 NMR, DSC and tensile test measurements. Only poly(butylene terephthalate) (PBT)-like diffraction pattern was observed in WAXD; however, two different C-13 spin-lattice relaxation time (T-1C) components were observed for aliphatic units, in which the longer and the shorter T-1C components correspond to the crystalline and the amorphous domains, respectively. Therefore the crystal structure of PBTSA was concluded to be formed by mixed crystallization of its comonomers. Such crystallization behavior enabled the PBTSA fibers to have well developed PBT-like crystal structure despite of its ideal randomness. Furthermore, due to the introduction of soft segments (BA and BS) into BT crystal lattice, melting temperature of PBTSA fibers (115degreesC) was over 100degreesC lower than that of PBT. (C) 2004 Elsevier Ltd. All rights reserved.