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A dislocation bow-out model has been developed to explain the strength of ultra-fine grained (UFG) materials with grain size roughly between 20 nm to 500 nm. In the model, perfect dislocations are assumed to be nucleated at grain-boundary sources and bow out between two pinning points on a boundary. Yielding is considered to occur when a dislocation takes a semi-circular shape under applied stress. Statistical consideration is introduced to evaluate the most probable pinning-point distance as a function of grain size. Comparison with experimental results is made for fcc UFG metals. It is found that yield stress as well as thermal activation parameters can be explained reasonably by the present theoretical model.