This paper discusses applicability of the Hill's condition of stability to granular materials with the aid of numerical simulations performed in terms of the granular element method. It also presents a new version of the granular element method itself before showing results of stress-probe simulations which give detailed behaviors of a specimen model. Results in a stable state show that the non-elastic behavior exhibits incrementally non-linear character which brings deviation from the non-associated flow rule in such a way that the Hill's condition is fulfilled. Other results show that applicability of the Hill's condition is restricted when the heterogeneity is progressing. The latter restriction seems to have its origin among premises appearing in a new derivation of the Hill's condition of stability.