ZnO nanorods were fabricated by ultrasonic treatment before and after a hydrothermal process. The morphology and structure of the nanorods were individually characterized by scanning electron microscopy and X-ray diffraction. The results show that before the hydrothermal process, fore-ultrasonic treatment can directly gain ZnO nanorods which mainly experienced four conversion stages from initial bulk Zn(OH)2, a coexisting phase of bulk Zn(OH)2 with ZnO nanoslices, ZnO nanoslices with flower-like ZnO nanorods and finally to purely flower-like ZnO nanorods. After the hydrothermal process, the post-ultrasonic treatment mainly influences the aggregation degree of the ZnO nanorods. The formation mechanism of ultrasonic treatment on ZnO nanorods is also discussed.