Insearch of new organic nonlinear optic crystals, aromatic compounds with donor and acceptor substituents are extensively studied. Some new crystals with large nonlinear coefficient have been reported. This kind of crystals is usually not useful in ultraviolet regoion, because the distance between energy levels in conjugated π bonds gets smaller due to the strong conjugation effect. In exploring new organic nonlinear optic crystals used in ultraviolet region, we have discovered a new phase matchable nonlinear optic organic crystal -- L-arginine phosphate monchydrate. Of the size 13X35X35mm^3 LAP single crystals with good optical quality have been grown by the cooling and evaporating methods. LAP crystal belongs to monoclinic system; its space group is P21. It shows the following properties: (1) transparency range over 10% extends from 230nm to 1940nm; over 50% extends from 240nm to 1500nm; (2) when basic wave is 1064nm and harmonic wave is 532nm, the calculated phase matching curved surface belongs to Hobden's ninth type; (3) SHG intensity of LAP crystal in phase matching direction (\w\=57, \t\=-13, phase matching type II) is 3.5 times greater than that of KDP crystal (\w\=59, \Ps\=0, type II). It is worth to note that this direction of LAP crystal will not be one of its maximum effective nonlinear coefficient; (4) when basic waves are 1064nm and 532nm, the sum-frequency effect has been observed; (5) the optical damage threshold is higher than 1 GW/cm^2.s at 1064nm; (6) the stability towards atmospheric moisture is excellent. As a result, LAP crystal is a promising nonlinear optic crystal.

Key words: PHOSPHATE, ORGANIC CRYSTALS, ARGININE, NON LINEAR OPTIC CRYSTAL