Microstructure and defects in nonpolar ZnO (1 1 (2) over bar 0) films with different thicknesses were studied by transmission electron microscopy. The ZnO films were grown on Al2O3 (1 (1) over bar 0 2) substrates by plasma-assisted molecular beam epitaxy. The misfit dislocations were observed at the interface with regularly spaced configurations, which were well agreed with the equilibrium spacing of the misfit dislocations calculated based on the lattice misfits. The diagonal defect along the ZnO [1 0 (1) over bar 0] and [0 1 (1) over bar 0] directions and the misfit dislocations were mainly observed in the 30 nm-thick ZnO film. As increasing the film thickness. the diagonal defect was seldom observed and the threading dislocations (in addition to the misfit dislocations) were being the major defects. The dislocation densities of the 240 nm-thick ZnO film were determined to be similar to 7.3 X 10(10) cm(-2) for the dislocations with [0 0 0 1] Burgers vector and similar to 6.1 X 10(9) cm(-2) for the 1/3[1 1 (2) over bar 0] Burgers vector, resulting in the total dislocation density of similar to 7.9 x 10(10) cm(-2). In addition to the perfect threading dislocations, stacking faults on (0 0 0 1) planes were observed. The type of stacking fault was determined to be a type-I-1 intrinsic stacking fault having the stacking sequence of (AB'ABC'BC), which has the Frank partial dislocations with the Burgers vector of 1/6[0 2 (2) over bar 3] at the end. The stacking fault density of the 240 mm-thick ZnO film was determined to be similar to 1.2 X 10(5) cm(-1). (C) 2008 Elsevier B.V. All rights reserved.