Sonochemical synthesis can be a facile, fast, efficient, versatile and economical way to prepare a large variety of conventional or novel nanostructured materials (metallic, magnetic, semiconducting, polymeric, etc.). In this work, zinc oxide nanocrystals were synthesized by irradiating and heating at 90 °C in a commercial ultrasonic bath a water solution of zinc nitrate hexahydrate and ammonia solution or hexamethylenetetramine as base catalysts. The evolution of the powder morphology and its crystalline structure was investigated at different times of ultrasonic irradiation (0-9 hours) and compared with samples obtained only by heating the solutions in a muffle furnace in order to enlighten the growth mechanism. It resulted that: i) the crystal morphology depends on the selected base, ii) for samples obtained by using the ultrasounds the homogeneity of the powders depends on the time of irradiation, iii) by comparing all samples obtained at 7 hours of heating, the aspect ratio of the crystals is higher for those which underwent also to ultrasounds.