The microstructure of polymer matrix under cylindrical confinement is key to understanding the size-dependent thermomechanical behavior of electrospun nanofibers. Coarse-grained molecular dynamics simulation was applied here to probe polymer systems under cylindrical confinement, prepared with or without pre-stretching. Simulation results showed that below a certain radius, a noticeable increase of the elastic modulus is observed with the decrease of the radius of cylindrical confinement. This size-dependent mechanical behavior correlated to the degree of polymer chain orientation. Modulation of density and bond orientation in the radial direction was observed: the density and bond orientation began to oscillate, increasing the oscillation amplitudes with decreases in the radius. Such behavior suggests that the cylindrical confinement enhances the bond alignment of the entire fiber and not in the near-surface layers only. The unstretched fibers had uniform density distribution along the fiber axis, while the stretched fibers demonstrated a fluctuation in density distribution. The crossover radius of size-dependent behavior was two orders of magnitude smaller than observed in real experiments, demonstrating that the confinement affects some internal fiber scale, which exceeds the scale of individual macromolecules, and this internal scale may be related to supramolecular structures of the polymer matrix rather than the individual macromolecules. (C) 2017 Wiley Periodicals, Inc.