The variation of hardness on (1 0 0) faces of orthorhombic ammonium sulphate crystals, AS (NH 4) 2SO 4, with load were studied. Vickers hardness numbers, H v, were found to increase with the increase in load. Work-hardening index ‘ n’ was found to be greater than 2 showing soft-material characteristics. Kick’s law has been satisfied after making a correction in the observed diagonal length of the indentation impression. The fracture toughness values ‘ K c’, determined from measurements of crack lengths, were estimated to be 0.078 and 0.073 MN m −3/2 for (1 0 0) faces at 90 and 100 g, respectively. The brittleness indices ‘ B i’ were estimated as 1.79 and 2.04 μm −1/2 for the same faces at same loads. Using Wooster’s empirical relation, elastic stiffness coefficient, c 11, has been calculated from hardness value.