We investigate post-annealing effects using an epi-GaN substrates for ZnO thin film growth by pulsed laser deposition (PLD). The growth of ZnO nanorods on a Si(100) substrate through a two-step process, annealing and off-axis PLD, without a metal catalyst is demonstrated as well. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were measured and the post-annealed ZnO films grown at Tg = 700°C had very smooth surfaces and the rms roughness was about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epi-layer and GaN/sapphire substrates. It was evident by AFM that growth temperature of 700°C helps the films grow in a step-flow growth mode. It was confirmed by cathode luminescence (CL) spectrum that the ZnO film grown at 700°C had very low visible luminescence, resulting in a decrease of the deep level defects. In the case of ZnO nanorods, controlling growth parameters during deposition enabled the adjustment of the dimensions of nanorods. The diameters of the grown nanorods ranged from 50 to 700 nm and the lengths are from 2 to 10 μm. The CL spectra were used to evaluate the states of defects within the ZnO nanorods. According to the CL results, the thinnest nanorod arrays were found to have fewer defects, while more defects were introduced as nanorods became thicker.