The online ultrasonic film casting process to manufacture nanocomposite films was developed. In this process, polycarbonates (PC) of low (LPC) and high (HPC) molecular weights were mixed with carbon nanotubes (CNT) and cast into films. Due to the lower viscosity of LPC than that of HPC, torque and pressure of extruder for LPC was lower, than those for HPC. Numerical and experimental investigations of necking phenomenon were carried out for film casting of PC/CNT composites. Experiments indicated that at the same processing conditions, the film width of HPC was larger, than that of LPC. The necking along film line decreased with imposition of ultrasound and CNT content, indicating that incorporation of CNT and imposition of ultrasound restrained the elongational flow behavior of melt, resulting in film of a larger width. Isothermal and nonisothermal numerical simulations of the process were performed. In isothermal simulations, the polymer melt was assumed to maintain the temperature of the die. In nonisothermal simulations, the temperature change along the film line was determined from heat transfer calculations with the WLF temperature-dependent viscosity. The simulated and experimental results on normalized film width, defined as a ratio of cast film width to die width, as a function of the distance from the die at various extension ratios were compared. The comparison indicated that changes in film width and thickness along the stretching direction in the nonisothermal process was in better agreement with experimental results than that in the isothermal process. Both experimental and simulated results showed a decrease of film width with take-up speed. Due to the presence of edge bead in the film, HPC film width was lower than the simulated one. With incorporation of CNT, a better agreement between experimental and simulated results was obtained, possibly due to reduced edge bead formation in the film.