Hardness is most likely to mean the resistance to indentation, and to the design engineer it often means an easily measured and specified quantity which indicates something about the strength and heat treatment of the metal. Especially, Vickers hardness is one of the most widely useful methods to obtain mechanical properties of a product. Firstly, in this study, a method to estimate hardness will be presented using FE simulations of Vickers hardness tests from the viewpoint that hardness indicates resistance to plastic deformation. To verify our method, the results of the simulations for several materials such as commercial aluminum alloy and steel will be compared with those of Vickers hardness tests for the materials. Secondly in this study, hardness numbers of the several materials will be obtained as a function of hydrostatic stress. Through the results of this study, the estimation of hardness number of a specific material will be very easy to obtain and access even though the material is under a kind of hydrostatic stress state.