It is proved theoretically in this paper that the field patterns of the traveling modes in a weakly guiding optical fiber are quite similar with those of the fiber-emitted modes in the far-field region. Based on our results, far-field peak intensities of the diffraction-added lobes observed on a screen are 2-3 orders of magnitude smaller than those of mode patterns. Therefore, it is possible to determine the traveling mode in a fiber from its far-field pattern.