ZnO nanostructures were formed via a hydrothermal reaction mechanism between simple anionic (ammonia orsodium hydroxide) and cationic (zinc acetate dehydrate) precursors without using any organic templates. Effect of the reaction conditions including the initial solution pH, concentration and type of the anionic and cationic precursors, and the reaction time and temperature on the nanostructure particle size and morphology were investigated. The nanostructures formed were analyzed by powder X-ray diffraction, energy dispersive X-ray analysis, and scanning electron microscopy. According to the results, the morphology of the nanostructures is highly pH-dependent. Needle-like nanostructures were formed using ammonia at initial solution pH value around 9 but plate-like nanostructures were formed using NaOH at pH value around 13 regardless of the reaction time or temperature. Varying the precursor concentrations could not be considered as an independent parameter per se due to the consequent changes impress the reaction medium pH which affects morphologies in turn. In general, increasing the reaction time and temperature increased the mean particle size of the nanostructures with no significant change in their morphology. It was found that the nanostructure morphology changes from nanoneedles to star-like ones due to higher addition rate of ammonia.