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Numerical simulations of forming limit diagrams (FLDs) are performed based on a rate-sensitive polycrystal plasticity model together with the Marciniak-Kuczynski (M-K) approach. Sheet necking is initiated from an initial imperfection in terms of a narrow band. The deformations inside and outside the band are assumed to be homogeneous, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the polycrystal model needs only to be applied to two polycrystalline aggregates, one inside and one outside the band. Each grain is modeled as an fcc crystal with 12 distinct slip systems. The response of an aggregate comprised of many grains is based on an elastic-viscoplastic Taylor-type polycrystal model. With this formulation, the effects of initial imperfection intensity and orientation, initial distribution of grain orientations, crystal elasticity, strain-rate sensitivity, single slip hardening, and latent hardening on the FLD can be assessed. The predicted FLDs are compared with experimental data for the following rolled aluminum alloy sheets: AA5754-