A new microcrack based continuous damage model is developed to describe the behaviour of brittle geomaterials under compression dominated stress fields. The induced damage is represented by a second rank tensor. The effective elastic compliance of damaged material is obtained from the Gibbs free energy function. The damage evolution is determined from a microcrack propagation condition. The proposed model is applied to the granite of Vienne. Comparison between test data and numerical simulations shows that the proposed model is able to describe main features of mechanical behaviours observed in brittle geomaterials under compressive stresses.