Zinc oxide (ZnO) nanowire synthesized from direct Zinc (Zn) vapor transport in O-2 environment has been studied. The results show that the first step is the formation of ZnO film on the substrate. Then anisotropic abnormal grain growth in the form of ZnO platelets takes place. Subsequently, single-crystalline ZnO platelets grow in [0001] direction to form whiskers. During whisker growth, transformation from layer-by-layer growth to simultaneous multilayer growth occurs when the two-dimensional (2D) Ehrlich-Schwoebel (ES) barrier at the ZnO island edge is sufficiently large and the monolayer island diameter is smaller than the island spacing. As multilayered islands grows far away from the base, isotropic mass diffusion (spherical diffusion) will gradually displace anisotropic diffusion (linear diffusion), which contributes to the formation of pyramid on the top plane of the whisker. When the pyramid contains enough atomic layers, the 2D ES barrier transits to 3-dimensional ES barrier, which contributes to repeated nucleation and growth of multilayered islands or pyramids on the old pyramids. The pyramids play a critical role to taper the whisker to nanorod with a diameter less than 100 nm. The nanorod then grows to nanowire via repeated growth of epitaxial hexagonal-pyramid shape-like islands on the (0001)-plane with {11 (2) over bar3} facets as the slope planes. During coarsening, the breakage of step motion of {11 (2) over bar3} facets and the appearance of {11 (2) over bar0} facets on the base of pyramids may result from the step bunching of {0001} facets, which is consistent with the existence of "2D'' Ehrlich-Schwoebel barrier on the edge of (0001) facets.