In the search for suitable non-linear optical (NLO) materials with higher non-linear responses and good optical, thermal, mechanical properties as well as fast growth rate, an attempt has been made to grow potassium dihydrogen phosphate (KDP) single crystals doped with L-arginine monohydrochloride (LAHCl). Slow cooling as well as slow evaporation methods were employed to grow crystals. The concentration of dopants in the mother solution varied from 1 to 10 M%. The solubility data for dopant concentration were determined and are compared with those of pure KDP. Nucleation studies show that metastable zone width of KDP is enhanced by the addition of LAHCl. The effect of additives on the growth, nucleation kinetics, structural, optical and mechanical properties has been investigated. With increasing additive concentration, the growth rate was found to increase. Vicker's microhardness study shows higher mechanical stability in LAHCl-added KDP crystals, the pure ones. Moreover, the addition of LAHCl improves the quality of the crystal surface and yields highly transparent crystals with well-defined features. (C) 2008 Elsevier B.V. All rights reserved.