Abstract: The inclusions in BBO single crystal can influence the ultimate crystal size as well as crystal properties. In this paper, the generation process of inclusions in BBO crystal by flux method is observed using Schlieren technique coupling with differential interference microscope.Results show that gaseous inclusions usually form at the inner part of perfect crystals and distribute near the interface. These indusions over generated originally from some small hexagonal cells. The formation and distribution of inclusions are significantly influenced by the development of crystal steps. The steps height and width may strongly constrain the develpment of inclusions, which results in the formation of inclusion clusters on the steps. The possible mechanism of inclusion generation is also discussed. It is noted that the main reason caused inclusions is the volatilization of some elements at high temperature.