Schistose and aciculate CuO nanostructures have been synthesized by a novel ammonia assisted hydrothermal method of copper alginate. The conversion processes of copper alginate are investigated by thermogravimetrics (TG) analyses under N2 and air atmosphere. The morphology, structure, and composition of the obtained CuO are investigated using SEM,TEM and XRD. It is found that different temperature and pH value resulted in the morphology and structure evolution of CuO. Ammonia was used as structure-directing agent in the hydrothermal system. The aggregation state of the nanostructures was controlled by the temperature. Dispersive schistose structures about 1μm in diameter were synthesized with 0.5mL ammonia at different temperatures. Dispersive microspheres of about 4 μm in diameter were also synthesized with 1 mL ammonia. Microspheres composed of nanoneedles and nanoplates were synthesized at 120°C and 160°C, respectively. Moreover, a possible growth mechanism governing the formation of such a nanomicrostructure was primarily discussed.