Zinc oxide (ZnO) nanorods with a-axis orientation were grown on a Si(100) substrate by a hydrothermal method, following which a post-annealing process was carried out at various temperatures ranging from 500 to 900 degrees C in vacuum. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) were carried out to investigate the structural and optical properties of the a-axis oriented ZnO nanorods. The XRD pattern of the a-axis oriented ZnO nanorods shows three diffraction peaks at 31.84 degrees, 34.48 degrees, and 66.43 degrees, corresponding to ZnO (100), ZnO (002), and ZnO (200), respectively. The texture coefficient (TC) ratio of the a-axis to the c-axis is increased with the annealing temperature. The residual tensile stress of the a-axis oriented ZnO nanorods is increased and the bond length is slightly decreased with increasing the annealing temperature. The near-band-edge emission (NBE) peak of the a-axis oriented ZnO nanorods is blue-shifted by the annealing process.