Electrospinning experiments for solutions of two polymers with different crystallization rates, polylatic acid (PLA), and nylon 6 solutions have been carried out. Hexafluoroisopropanol (HFIP) was used as a common solvent to dissolve both PLA and nylon 6. Using the fact that nylon 6 has the faster crystallization rate than PLA, the effect of thermal annealing conditions on the crystal structures of two electrospun fibers has mainly been studied via thermal and mechanical analysis. First, optimal conditions for the formation of uniform nanofibers have been determined by observing their morphology and rheological properties. Thermal analysis revealed that PLA as-spun fiber exhibits cold crystallization due to insufficient crystal growth in spinline. Also, the effect of thermal annealing on the structural change of nylon 6 fibers including the degree of flow-induced crystallization is totally different from that of PLA fibers. Finally, tensile measurements of annealed fibers show that the ultimate tensile strength and Young's modulus gradually increases with increasing annealing time for both PLA and nylon 6 mats, resulting from the further growth of crystal structures. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 752-758, 2011