The authors report for the first time the size dependency of the elastic modulus of well-aligned single polymeric nanofibers. The nanofibers were fabricated from electroactive polymers (EAPs) and had an ellipsoidal cross section because of impingement between a solid surface and a polymer jet during electrospinning. Although the EAPs had very weak mechanical properties in the bulk, the elastic modulus of single EAP nanofibers increased exponentially as the diameter of the EAP nanofibers decreased to diameters of a few tens of nanometers. The elastic modulus of single nanofibers was measured using three-point bending tests employing an atomic force microscope. (c) 2006 American Institute of Physics.