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 <sup>1</sup>H 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 P<sub>212121</sub> with cell parameters : a = 15.657(10) Å, b =16.385(4)Å, c = 7.491(15) Å, α = 90<sup>o</sup>, β = 90<sup>o</sup>, γ = 90<sup>o</sup>. Its bulk crystal of the size 39x 41x14mm<sup>3</sup> is grown from acetone solvent by slow evaporation technique.