Oriented growth of KH2PO4 (KDP) under a compressed Langmuir monolayer of arachidic acid (AA) has been studied by using pi-A isotherms, X-ray diffraction (XRD), Brewster angle microscope (BAM), and atomic force microscopy (AFM) techniques. All produced crystals are almost connected with the monolayer by the crystal face (100), epitaxial forming multilayer structures along the vertical direction. BAM observation shows that KDP crystals induced by the Langmuir monolayer are nearly homogeneous and uniform in crystal size. Crystallization induction time is reduced evidently. The optical second harmonic generation (SHG) experiment indicates that the SHG signal of the sample of 25 layers deposited in Z type is better than that of the crystals in the absent of a monolayer, which demonstrates that oriented KDP crystals are homogeneous with a uniform orientation.