The Plasticity of single crystals MgSiO3 perovskite was studied (at room temperature) by the microhardness tests using a Vickers and a Knoop indenter. The Vickers hardness, Hv of MgSiO3 perovskite was determined to be 18 +/- 2 GPa, about a factor of two larger than the hardness of either enstatite or olivine, indicating a significantly larger resistance to plastic flow of this mineral than that of the upper mantle minerals. The normalized hardness, Hv/&mgr; (&mgr;: shear modulus), of MgSiO3 perovskite is in reasonable agreement with those of titanate perovskites (SrTiO3, BaTiO3, CaTiO3), although a cubic perovskite SrTiO3 has a significantly lower hardness than others. The Knoop tests showed anisotropic hardness consistent with the slip systems [111>(112¿) and [11¿1>(11¿2¿). ¿ American Geophysical Union 1990