Ultrafine fibers produced by electrospinning often exhibit bead-on-string structures, which have generally been considered to be undesirable "by-products" or defects. Theoretical analysis in the literature predicted three types of instabilities for an electrically driven jet the axisymmetric Rayleigh instability, the electric field-induced axisymmetric, and whipping instability. The process of bead formation revealed that the formation of a beaded structure resulted from axisymmetric deformation and flow of the jet. Applied voltage, solution surface tension, and conductivity (or jet charge density carried by the moving jet) were theoretically demonstrated to be important for jet axisymmetric instabilities. Experimental results revealed that these parameters influenced the formation of beaded fibers in the same manner as they did for the axisymmetric instabilities. As a result, the axisymmetric instabilities were considered to be the most likely mechanism of beaded fibers formation during electrospinning. © 2006 Society of Plastics Engineers.