6-Methoxy naphthaldehyde is prepared by the Grignard reaction of 2-bromo 6-methoxy naphthalene with triethylorthoformate. The formation of the compound is confirmed by recording IR and 1H NMR spectra. The SHG frequency conversion efficiency of this compound was measured by powder technique using Nd: YAG laser. This compound showed NLO property 0.59 times that of urea. The crystal system is found out by single crystal X-Ray diffraction studies and the crystal system found to be orthorhombic and space group P212121 with cell parameters : a = 15.657(10) Å, b =16.385(4)Å, c = 7.491(15) Å, α = 90o, β = 90o, γ = 90o. Its bulk crystal of the size 39x 41x14mm3 is grown from acetone solvent by slow evaporation technique.