Gas Jet Fiber (GJF) process involves aerodynamic forces to draw a jet of polymer solution from a nozzle and to convert it into polymer nanofibers. The jet of polymer solution undergoes rapid stretching and turns into solid nanofibers as the solvent evaporates. The nanofibers are collected as non-woven mat. An important parameter defining the nanofiber properties is fiber diameter. This work investigates selection of diameter of nanofibers from homogeneous solutions of two immiscible polymers in two mutually miscible solvents. Several morphological forms such as interpenetrating network, bilobal, and core-shell are obtained by selecting two solvents with different vapor pressure and solubility parameters. This paper addresses the roles of polymer solution viscosity and solvent volatility in defining polymer fiber diameter varying between 1 m and 100 nm.