We report a new direct fabrication of the ZnO nanorods (NR) by hydrothermal method, in which the preparation of seed layer is eliminated. We show that the tuning of initial temperature rate during the hydrothermal process plays a key role in the structural modification of the ZnO NR. A highly oriented ZnO NR is successfully fabricated by using a low rate of initial temperature. The increase of optical absorption and electron transport was obtained by reducing the diameter and increasing distribution of the ZnO NR on the substrate. Interestingly, an additional absorption from the defects is obtained in the system, which plays an important role in expanding the optical absorption. Our system will provide a favourable characteristic for developing the high-performance optoelectronic devices with high optical absorption and high electron transport.