The initial growth behavior and resulting microstructural properties of heteroepitaxial ZnO thin films prepared by pulsed laser deposition on sapphire (0001) substrates were investigated. High-resolution x-ray diffraction and transmission electron microscopy studies revealed that the initial growth behavior and the microstructure of the films were significantly dependent on the growth parameters employed. ZnO films grown at 700 °C with an O2 partial pressure of 20 mTorr initiated in a columnar growth mode and contained two types of domains. These domains were in plane orientated either ZnO [11 2- 0] ∥ Al2 O3 [10 1- 0] or ZnO [10 1- 0] ∥ Al2 O3 [10 1- 0] and were surrounded by highly defective domain boundaries with threading dislocations. ZnO films grown at 800 °C with 1 mTorr O2 showed a two-dimensional layered growth with only one in-plane epitaxial relationship, ZnO [11 2- 0] ∥ Al2 O3 [10 1- 0]. Most of the defects in the layered grown films were basal plane stacking faults near the interface between the ZnO and the substrate. The mechanism of formation of the 30°-twisted domains with the in-plane orientation of ZnO [10 1- 0] ∥ Al2 O3 [10 1- 0] is discussed.