Microstructures of as-grown and annealed beta-Ga2O3 single crystals were studied by electron microscopy. Energy filtered electron diffraction patterns exhibited weak diffuse scattering at 1/2 and 1/4 positions between the fundamental reflections along the q (20)((4)) over bar vector. Dark-field images observed with the 1/2 diffuse scattering showed a characteristic domain-like structure, where the size of each domain was less than 5 nm. The diffused scattering was somewhat weakened by annealing the specimen. The observations indicate a non-homo geneous structural modulation, which is considered to be related to the distribution of oxygen vacancies. (C) 2002 Elsevier Science Ltd. All rights reserved.