Recent studies show that polyethylenimine (PEI) benefits the hydrothermal growth of ZnO nanorod arrays due to its effect on chelating Zn(2+) ions and selectively absorbing on the crystal facets. In the present work, PEI is found to play more complex roles during the growth. The observation on the solution color and the UV-VIS and FTIR spectra demonstrate a Mannich reaction between HCHO and -NH2 groups of PEI during the hydrothermal procedure. -N=CH2 groups produced by the Mannich reaction contribute to the color change. The HMT decomposition is the controlling step for the whole growth and the decomposition rate depends on the hydrothermal temperature and the PEI addition amount. Increasing the PEI amount elevates the hydrothermal temperature to 130 °C and as a result the crystal quality is improved. A stepping heating process is designed to grow ultralong ZnO nanorod arrays by optimizing the solution composition. The rod length reaches to about 13 μm by one growth circle. The mechanisms associated with PEI addition provide a novel route for synthesizing metal oxide nanomaterials, particularly those requiring the higher hydrothermal temperature.