Constitutive models based on dominant mechanisms in hot forming are proposed. These models consider inter-granular deformation, grain boundary sliding, grain boundary diffusion and grain growth. New stress-strain rate relationships are proposed to predict deformation due to grain boundary sliding and grain boundary diffusion. Beside a Taylor type polycrystalline constitutive model, a visco-plastic relation in conjunction with a yield function is used to predict inter-granular deformation with much less computational costs. The proposed models are calibrated with tensile test data of AA5083 at . The calibrated models closely fit simple tension experimental data for various strain rates and strains. Then as an example the models are used to simulate a tray forming experiment. Dome heights and tray thicknesses at various positions during forming time can well predict experimental observations.