The atomic and electronic structures of [0001]/((1) over bar(2) over bar 30) Sigma 7 symmetric tilt grain boundary in an undoped ZnO bicrystal were investigated by high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. HRTEM imaging and atomistic calculations revealed that the grain boundary was composed of at least two types of structural units. It was also found that one of the structural units has two threefold-coordinated atoms per a unit and the other has two fivefold-coordinated atoms. First-principles calculations indicated that these atoms with various coordination numbers do not form deep unoccupied electronic states in the band gap of ZnO, which is in consistency with a linear current-voltage characteristic observed for the bicrystal with the Sigma 7 boundary. (c) 2005 Springer Science + Business Media, Inc.