Defects and structural characteristics (threading dislocations and angles of rotational disorder in the subgrain) of wurtzite-type ZnO films grown by electron cyclotron resonance-assisted molecular beam epitaxy on (0001) c-plane sapphire have been investigated extensively using conventional transmission electron microscopy and high-resolution electron microscopy (HREM). Through the cross-sectional and plan-view observations, the existence of threading dislocations in the ZnO film and the basic crystallographic orientation relationship of (0001)(ZnO)\ \ (0001)(sapphire) and [2 (1) over bar(1) over bar0](ZnO)\ \ (1 (1) over bar 00)(sapphire) were clarified. The line directions of most threading dislocations were found to be normal to the interface. The density of the threading dislocations in ZnO film was estimated to be 1.9x 10(11) cm(-2) and the subgrains being accompanied by the threading dislocations and Burgers vectors of 1/3(11 (2) over bar0) were clearly observed. It was found that the size of the subgrains ranges from 15 to 150 nm and the subgrains are rotated by 1 degrees -5 degrees with respect to the ZnO film. The overlapping manner of the ZnO film and the sapphire substrate in the subgrains was discussed taking into account the moire fringes of HREM images.