Zinc oxide (ZnO) nanostructures were synthesized by a simple sonochemical method without the assistance of an additional surfactant. The morphologies can simply be controlled only by changing the addition sequence of NH3.H2O. When NH3.H2O was added before or at the beginning of the ultrasonic treatment, a flower-like nanostructure and ellipsoidal particles self-assembled by nanorods were obtained, respectively. The strong visible emission implied high atomic defects. The emission intensity was dependent on the addition sequence of NH3.H2O greatly.