The variation of hardness on (1 0 0) faces of orthorhombic ammonium sulphate crystals, AS [(NH4)(2)SO4], 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 mum(-1/2) for the same faces at same loads. Using Wooster's empirical relation, elastic stiffness coefficient, cll, has been calculated from hardness value.