Mechanical fatigue crack nucleation and propagation is modeled in bimodal grain size aluminum alloy. A multiscale modeling approach in conjunction with a continuum based damage modeling technique, successive initiation, is used to determine microstructural site of crack nucleation and its propagation through different regions of the materials. Analyses conducted for material with different coarse grain volume ratios under different load amplitudes showed that damage initiates at the interface of coarse grains and the ultrafine grain matrix. It propagates initially through coarse grains with higher initial damage rate. Once the coarse grains lose their load bearing capacity, the load is transferred to the ultrafine matrix and it fails rather quickly. Comparison between different large grain volume ratios shows that the small distance between large grains at high coarse grain volume ratios facilitates crack bridging between coarse grains and results in very high crack propagation rate in coarse grains which eventually results in catastrophic failure of the whole structure.