The sharp increase in elastic modulus of electrospun polymer nanofibers with decrease in their diameters is now a well-known phenomenon. Unfortunately, up to now, the physical reasons resulting in the above size-dependent behavior are unclear. The proposed explanation is based on the confinement concept. A manifesting mechanism of the confinement effect which provides the size-dependent elastic modulus of electrospun polymer nanofibers is discussed. According to this model, the nanofiber polymer matrix contains anisotropic regions consisting of correlated worm-like subchains, partially oriented along the fiber. A fiber elongation is accompanied by relative rotations of the above regions. Confinement effect is that these rotations are hindered by the fiber boundary. As a result, the elastic modulus depends on the fiber diameter. This restriction is dominant for the thin fibers, is decreasing with increase of the fiber diameter, and becomes negligible for thick fibers. Such a behavior is in good agreement with experimental observations. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013, 51, 756-763