The role of hexamine in the growth of 1D ZnO nanostructures, so far is only assumed, unpredictable and not well explained. Further, the role of hexamine in the growth of 1D ZnO nanostructure by hydrothermal method, especially at the higher concentrations (0.08 M and above) has not yet been well reported. The present study is designed to investigate the role of hexamine in lateral and vertical side growth of ZnO nanorods with different concentrations (0.08 M and above) of hexamine by keeping the concentration of zinc nitrate hexahydrate at 0.1 M. The influence of hexamine in the hydrothermal growth of ZnO nanorods is studied with various characterization tools. The results show that hexamine strongly influence the morphology of ZnO nanostructure and single rod structure transforming into multiarmed structure with increase in hexamine concentration. The growth mechanism of multiarmed ZnO nanorods with respect to hexamine is effectively investigated and discussed in detail from the basic nucleation theories. Lateral growth and merging of rods on the surface is evidenced at the higher concentration of hexamine.