Highly uniform mono-dispersed ZnO sub-microrods were synthesized under the condition of a limited injection of Zn divalent ions into an aqueous solution of hexamethylenetetramine at 85 °C. The mechanisms governing the initial seed formation and growth of the ZnO sub-microrods were revealed as an oriented aggregation and classical growth, respectively, from an analytic observation with a high-resolution transmission electron microscope. To demonstrate the capability and possibility of forming uniform microrods for electronic applications, a monolayer of thermally annealed rods was produced on a flexible substrate.