Nanoporous ZnO film was prepared by the stepwise solution route: the ultrasonic irradiation mediated successive ionic layer adsorption and reaction (SILAR) method. The growth of ZnO submicron particles in the film was arrested in the immature status by reducing the deposition cycle from 50 to 30. The morphology and microstructure of individual ZnO submicron particle was analyzed by scanning electron microscope, transmission electron microscope and electron diffraction. Results reveal that ZnO submicron particle was constituted by numerous nanocrystallites with size of 3-10 nm, and exhibited ordered nanostructure preferably along (002) plane. The formation mechanism of specific submicron particle on the nanometer level was discussed, emphasizing the effect of ultrasonic irradiation on the nucleation and growth of ZnO nanocrystals in zinc-ammonia aqueous system. In addition, effects of NH3-Zn ratio and pH value in the precursor were examined, which has established the suitable chemical environment for the formation of dense ZnO submicron particles on substrate, and especially provided further experimental proof for the function of ultrasonic irradiation on promoting the nucleation on submicron particles. (c) 2006 Elsevier B.V. All rights reserved.