In this paper the effects of 13 material and operating parameters on electrospun fiber diameters are determined by varying the parameter values in an electrospinning theoretical model. The complexity of the electrospinning process makes empirical determination of the effects of parameters very difficult. The results show that the five parameters (volumetric charge density, distance from nozzle to collector, initial jet/orifice radius, relaxation time, and viscosity) have the most significant effect on the jet radius. The other parameters (initial polymer concentration, solution density, electric potential, perturbation frequency, and solvent vapor pressure) have moderate effects on the jet radius. Parameters relative humidity, surface tension, and vapor diffusivity have minor effects on the jet radius. Knowing the relative effects of parameters on jet radius should be useful for process control and prediction of electrospun fiber production.