A mathematical model was developed to simulate an optical fiber-forming process which is based on internal diffusion and surface evaporation (IDSE) of monomers in a liquid jet. The IDSE process requires that a polymer solution which consists mainly of one type of monomer and a different polymer be prepared and then extruded through a die to form a circular liquid jet, the monomers will diffuse to the jet surface and then evaporate. Once the concentration distribution of the monomers in the jet reaches a desired pattern, photopolymerization will commence and an optical fiber is generated. The model that describes the IDSE process involves momentum, heat and mass transfer with free-surface hows. Methods for evaluating the physical parameters in the model were proposed. An advanced numerical technique based on the finite-element method incorporated with schemes to treat free surfaces and high Peclet numbers was developed for the solution of the mathematical model. A test case was selected for illustration; the effects of several variables on the fiber forming were examined. It was found that only by balancing the internal diffusion and the evaporation at the jet surface could an optical fiber with a desirable pattern of the refractive index be generated.