Average intensity and average contrast distributions of the far-field interference patterns of laser light emitted from end faces of optical fibers are theoretically studied with the consideration that mode conversion takes place in the optical waveguide. The optical fiber considered here is a multimode graded-index fiber having random fluctuations of the refractive index in the core as well as in the cladding. The index fluctuations are assumed to obey a normal probability distribution so that the fourth moment of the radiation field can be obtained in terms of the second moment. Using the properties for the index fluctuations, average contrast, defined as a normalized deviation of the speckle intensity, is calculated within the second-order approximation of perturbation. The results show that the average contrast decays with lateral distance in the far-field plane, and its decreasing tendency is rather sharp in the off axis when the fluctuation scale of the refractive index becomes small.