Abstract: This paper presents a dynamic investigation of the polycaprolactone (PCL) nanofibers of diameters in the 100-500 nm range that were fabricated via electrospinning. We conducted AFM-based strength experiments involving a single-strand fiber and found that its elastic modulus exhibits a strong size dependency after its diameter has been reduced past a threshold size. In an attempt to explain this inverse behavior, we developed a strain gradient vibration model and used it to analyze a clamp-clamp nanofiber employed in our experimentation. As a check, our new vibration equation reverts back to the classic Euler-Bernoulli equation when strain gradient effects are neglected. To predict the onset of the inverse size-dependent response, we proposed a material length scale parameter leff and showed that at leff =78nm our theoretical predictions for the PCL nanofibers conform reasonably well to the experimental data.