Correlations between observable features of the droplet-jet shape and the diameter of electrospun nanofiber were studied. Three shape parameters derived from optical images of the jet transforming from a droplet were identified. They are "L-R curvature", "initial jet diameter", and "transition slope". Nanofiber diameter increased as the observed value of each shape parameter increased. The correlation between "initial jet diameter" and fiber diameter, as well as the correlation between "transition slope" and fiber diameter, were not affected by the variation of voltage or concentration of the polymer solution. "Initial jet diameter" and "transition slope" have potential applications in online controlled electrospinning to predict nanofiber diameter as real-time feedback information to guide the control of electrospun nanofiber diameter.