In this paper, we propose a unified approach for modelling of elastic-plastic and viscoplastic behaviour coupled with induced damage in quasi-brittle rocks. The general framework is first presented. Rock materials may develop plastic deformation at different time scale due to evolution of material microstructure and material viscosity. Both instantaneous and delayed plastic deformations are described within the unique constitutive model. Material damage induced by microcrack is coupled with plastic deformation. The proposed model is applied to two typical materials. Comparisons between numerical predictions and experimental data show that the proposed model is able to describe material response at different time scale. (C) 2008 Elsevier Ltd. All rights reserved.