Atomic structures of crystallographic shear planes (CSPs) in nanocrystalline thin films of semiconductor SnO2 were investigated by high-resolution electron microscopy. The films were prepared by electron beam evaporation in high vacuum (10(-6) torr) and followed by annealing in synthetic air at 700 degrees C for 1-2 h. CSPs with the displacement vector of [1/2 0 1/2] observed in the planes parallel to ((1) over bar 0 1), (1 1 0) and ((3) over bar (2) over bar 1). Most of the CPSs were found to terminate or interact with each other within SnO2 crystallites. Partial dislocations exist at terminal places of CSPs or along intersecting lines of CSPs. CSP steps were also observed. Structural models of these defects have been proposed. Based on analysis of experimental data, it has been suggested that the Sn/O ratio at CSPs which are not parallel to their displacement vector, at cores of partial dislocations and at CSP steps, is higher than that of the perfect structure, that is, these defects are able to provide extra free electrons with the films.