It has frequently been observed that the ductility or elongation of a material can significantly be affected by its grain size. Poor ductility with low elongation has been found in ultrafine-grained (UFG) steels when their grain size is smaller than a critical value. The so called instability of plasticity is a well known drawback for UFG steels and has greatly prevented them from wider application. Although the instability has been attributed to the lack of strain hardening capacity of ultrafine-grained materials, its mechanism is still unclear. In this paper, a newly developed model for predicting the dependence of uniform elongation on grain size is described, using which the instability of plasticity in UFG steels can be explained. The model is based on a strain hardening model for polycrystalline metals that was previously developed by the present author and was modified in the present study to take the grain size effect into account. The uniform elongation calculated using the model is in good agreement with that experimentally measured in two steels. It was found that the elongation begins to decrease rapidly when the grain size is reduced to below 2μm - a clear sign of the occurrence of the instability of plasticity.