Summary: Simulation of large deformation and post-failure of geomaterial in the framework of smoothed particle hydrodynamics (SPH) are presented in this study. The Drucker-Prager model with associated and non-associated plastic flow rules is implemented into the SPH code to describe elastic-plastic soil behavior. In contrast to previous work on SPH for solids, where the hydrostatic pressure is often estimated from density by an equation of state, this study proposes to calculate the hydrostatic pressure of soil directly from constitutive models. Results obtained in this paper show that the original SPH method, which has been successfully applied to a vast range of problems, is unable to directly solve elastic-plastic flows of soil because of the so-called SPH tensile instability. This numerical instability may result in unrealistic fracture and particles clustering in SPH simulation. For non-cohesive soil, the instability is not serious and can be completely removed by using a tension cracking treatment from soil constitutive model and thereby give realistic soil behavior. However, the serious tensile instability that is found in SPH application for cohesive soil requires a special treatment to overcome this problem. In this paper, an artificial stress method is applied to remove the SPH numerical instability in cohesive soil. A number of numerical tests are carried out to check the capability of SPH in the current application. Numerical results are then compared with experimental and finite element method solutions. The good agreement obtained from these comparisons suggests that SPH can be extended to general geotechnical problems.