A chain-of-bundles probability model is often used to predict the strength of unidirectional composites. In this model, it becomes necessary first to calculate stress redistribution due to fiber breakage. A rather simple estimation of this stress redistribution has hitherto been conducted. This paper examines it in a more precise manner by adopting a shear-lag assumption. Both ordinary composites and hybrid composites are analyzed. Two idealized models are chosen in the analysis: (1) an infinite model in which a group of broken fibers is embedded in an infinite number of continuous fibers, and (2) a repeating model in which broken fibers appear repeatedly. Actual load concentration factors will fall in between the above two extreme cases. Results of load concentration factors are presented in terms of the number of fractured fibers. The present analysis provides data for a statistical calculation of the strength of composites.