Bioactive monetite (anhydrous calcium hydrogen phosphate, CaHPO4) with orderly layered structure assembled by nanosheets has been successfully synthesized by a sonochemical-assisted method in the presence of cetyltrimethylammonium bromide (CTAB). The thicknesses of the nanosheets are 100-200 nm, and the lateral sizes are about 2 mu m. Because of the strong affinity with the phosphate ions as well as the (200) faces of the crystals, CTAB molecules can make the formation and stabilization of monetite nanosheets with (200) exposed face. Ultrasonic irradiation makes the transition from disordered state to oriented state before the oriented assembly of monetite nanosheets. The ultrasonic irradiation provides enough external work to make the assemble process possible in thermodynamics. The drastic flow stirred by the supersonic jet in the solution accomplishes the transition and successive oriented assembly of nanosheets in dynamics. This study would offer a simple method to design and synthesize oriented-assembled materials.