A two-phase flow model is developed to simulate the fiber motion in the nozzle of an air-jet spinning machine. The computational results coincide with the images captured by high-speed photography. Three parameters, the first nozzle pressure, the jet orifice angle, and fiber flexural rigidity, are discussed in terms of their influences on wrapper formation and, consequently, on yarn structure and yarn quality. The predictions of yarn tensile properties tally with the experimental results reported by several researchers.