Based on the concept of local thermal inconsistency, a constitutive model was developed for the description of the behaviors of metallic materials subjected to thermomechanical loading with fast heating, with reference to heating-assisted forming applications. The model considers effect of plastic deformation, of temperature as well as heating-rate and of recrystallization, on the mechanical property, hardening and damage of the materials. It can account for the reduction of the rupture strength of metallic materials subjected to high heating-rates or heating-rate histories, and the reduction of failure temperature of pre-stressed metallic materials heated at high heating-rates. The constitutive behavior of brass H62 subjected to uniaxial thermomechanical loading with fast heating is described and compared with experimental results. The satisfactory agreement between the computed and the experimental results shows the validity of the proposed model.