Modeling of strengthening by nanocrystalline materials need consideration of both dislocation interactions and sliding due to Coble creep acting simultaneously. Such a mechanism is considered in this paper. It is shown that a model based on using Coble creep (with a threshold stress) for finer grains and conventional Hall-Petch strengthening for larger grains, appears to be most successful in explaining experimental results provided a grain size distribution is incorporated into the analysis to account for a distribution of grain sizes occurring in most specimens. Use of an alternate formalism of Coble creep proposed recently gives a somewhat less satisfactory agreement with experiments.