In this paper, the viscoplastic regularization method is used to simulate strain localization in a compressed prismatical sample of Fontainebleau sandstone. First of all, the limits of Rice's formalism concerning the prediction of the localization band width and the post-bifurcation treatment are recalled. Then the theoretical aspects of the viscoplastic regularization method is reviewed. The non-objectivity or mesh-sensitivity of the solution is pointed out when a simple plastic model without viscosity is used. Finally, a fictitious viscosity parameter in the form of relaxation time is introduced in the model and regularizes the problem.