A constitutive model is proposed for modeling rate and temperature dependent stress-strain behavior of amorphous polymers. The model developed here is a modification of cooperative-viscoplasticity theory based on overstress (VBO) model. The modifications are performed to capture the temperature dependent behaviors below and through the glass transition temperature (Tg) of amorphous polymers as well as rate dependency from quasi static to high strain rate loading. In the proposed constitutive model, adiabatic heating due to high strain rate deformations is included to capture the high strain rate behaviors. The temperature dependent behavior below, around and through Tg will be captured with the modifications performed on the viscoplastic strain rate. The model has been shown to be predictive of compression behavior of DER 353 epoxy at quasi and high strain rates and a temperature range from room to Tg. The genetic algorithm (GA) parameter optimization procedure has been proposed to determine material parameters of the models. The model parameters are determined using GA optimization method. The newly modified model is able to capture the mechanical behavior below and through Tg and also all of the features of the large strain behavior, including the initial modulus, yield stress, strain softening and strain hardening.