Zinc oxide (ZnO) nanocrystalline with different morphologies was successfully fabricated by solid-state reaction using aging Zn as precursor. Systematic experiments were carried out to investigate the factors that affect the morphologies of the samples. X-ray diffraction (XRD) spectrum shows that the as-prepared sample was hexagonal wurtzite structure. Scanning electron microscopy (SEM) images indicate that ZnO with different morphologies such as comb-like, tetrapod and flake structures, could be controlled by simply varying the ambient pressure, gas mixture ratio and growth temperature. The gas sensitivity properties of ZnO nanocrystalline thin films are investigated by observing the change of capacitance and conductance with a planar capacitor. It is found that the comb-like ZnO thin film sensor exhibits high sensitivity and fast response to ethanol at 150 °C.