The authors analyze electromagnetic wave propagation through optical fibers consisting of a glass core surrounded by a high-refractive-index dielectric thin film, in turn surrounded by a glass cladding. The asymmetry of the structure due to different core and cladding refractive indices results in two types of guided modes: they propagate either in the core and film or only in the film. The dispersion equations are derived, and mode classifications are established for transverse and hybrid modes. The cutoff conditions of transverse modes are given. Numerical simulations are performed to obtain the effects of film index and thickness on the cutoff frequencies and dispersion curves. It is shown that a high-index film causes the modes to become evanescent in the core and guided only in the film. These modes form two types of linearly polarized modes, according as they follow the TE or the TM mode propagation curves of asymmetric slab waveguide. Investigation of power flow along the propagation direction shows that the higher the refractive index of the film, the higher the power density in the film.