Porous thermoplastic polyurethane (TPU) membranes were produced by the electrospinning process. Two different TPUs and their blends were used to investigate the effects of material composition, solution concentration, and rheological properties on the microstructure, fiber diameter, and fiber diameter distribution of the electrospun membranes. The ratios of hard and soft segments in the solutions were adjusted by varying the blend ratios of TPUs dissolved in N, N-dimethylformamide. The solutions with higher TPU concentrations and more hard segments exhibited a higher viscosity, larger storage and loss moduli, and greater electrospun jet stability. Solutions with concentrations around the critical chain entanglement concentration (C-e) produced bead or beaded fiber structures, while bead-free fibers of a uniform diameter were obtained when the concentration increased to about two times that of C-e. Relationships between the electrospun fiber diameter, the Berry number, and the normalized concentration of the solutions were studied as well.