A considerable number of new crystalline compounds were prepared by reacting L-arginine (L-Arg) with a series of inorganic and organic acids. In addition to the usual arginine slats with composition L-Arg-HX, where HX represents various acids, an entirely new class of crystal with the composition L-Arg 2HX was discovered. Due to the presence of the optically active L-arginine component, all these crystal had non-centro-symmetric structures. Most of the crystal prepared in this study gave strong phase-matched second harmonic generation signals with a Nd:YAG laser, and some appear quite promising for nonlinear optical applications.