Electrospinning has become a widely-used method for fabricating polymer nanofibers for various applications including filtration, drug delivery, and tissue engineering. Due to the high extensional forces during the electrospinning process, and the rapid crystallization and solidification during solvent evaporation, molecular orientation may develop within the resulting fibers. The properties of electrospun fibers are expected to be sensitive to level of orientation in the fibers. Various reports have shown an increased modulus with decreased fiber diameter, and molecular orientation has been used to explain this trend. However, there have been relatively few studies of the detailed relationship between fiber diameter and molecular orientation, especially at the single fiber level. Here we report a quantitative study of the orientation in individual electrospun poly(caprolactone) (PCL) and poly(L-lactic acid) (PLLA) fibers using low-dose electron microscopy and diffraction techniques. Our results confirmed that for electrospun fibers of PCL and PLLA processed under similar experimental conditions, the molecular orientation decreased as.the fiber diameter increased. The extent of orientation remained high for quite large fiber diameters, with azimuthal orientation of 20 degrees seen up to similar to 500 nm for PCL and similar to 2000 nm for PLLA. (C) 2017 Elsevier Ltd. All rights reserved.