Zinc oxide whisker (ZnOw) with a shape of tetra-needle was prepared, and the structures during the crystal growth were studied by SEM images. The cross section of hexagonal of the product and the gradually converting profile from a trigonal through a pentagonal and finally to a hexagonal sectional profile was observed. The leg of the product was proved to be composed of a single crystal of Wurtzite structure by analysis of electron diffraction pattern. The theoretical prediction and experimental study gave the same results that the aspect ratio (lengths to radicals) of the whiskers could be adjusted by controlling the reaction conditions. The crystal form and lattice constants of the whiskers were determined by electron diffraction and X-ray diffraction analysis.