The combined effect of pressure and temperature on the mechanical behavior of polyurea is characterized and integrated into a constitutive model based on Williams-Landel-Ferry (WLF) equation. An increase in pressure is related to a decrease in temperature, both pressure and temperature affecting the polymer's molecular free volume and thereby introducing a pressure-dependent term into the WLF equation. The model is derived based on Fillers and Tschoegl's original work, regarding the free volume as a state variable. The model provides a closed-form expression for the time scale shift factor for equivalent temperature in terms of temperature and pressure and its three-dimensional surface representation. The prediction of the present model agrees well with pressure shift factor data obtained from confined compression tests of polyurea, reported elsewhere, confirming our constitutive model and its experimental calibration procedure. A simplified model of the pressure effect is also developed which yields good approximate results. (C) 2016 Elsevier Ltd. All rights reserved.