The purpose of this study is to elucidate factors affecting the statistical variations typically observed in the tensile strength of natural plant fibers such as kenaf bast fibers (KBFs) which recently have been looked upon as promising reinforcing fillers of environmentally-preferable polymeric composite materials. A statistical finite element modeling framework by combining beam and solid elements was proposed with both the variable cross-sectional geometries and the meso-scopic internal organic structures stochastically considered. The present modeling strategy was then effectively applied to the stress analysis and strength simulations of natural plant fibers under axial tension. From numerical examples in the case of KBFs, it was shown that the meso-scopic internal organic structures, which consist of elementary fibrous cells (EFC) and inter-cell material (ICM), gave rise to axial stress concentrations in EFC and free-edge shear stresses in ICM, both of which should initiate the fiber fracture. Furthermore, by applying two-parameter Weibull analysis to the simulation data, the dependency of fiber strength and its initiating failure modes upon gauge length was also examined, which shows fairly good similarity with the actually-observed experimental results of monofilament tensile strength of natural plant fibers.