A size distribution measurement method of the oscillating cavitation bubbles using the diffraction pattern of the bubbles has been studied by the authors. This method can measure the diameter distribution of the small oscillating bubbles. However, it has a disadvantage in that the measurement result of the method is disturbed by the acousto-optic effect. In this study, the influence of the acousto-optic effect on the measurement was experimentally investigated. As a result, it was found that the diffraction pattern tended to be disturbed along the direction of the ultrasound propagation. It was also recognized that the disturbance could be reduced using the diffraction pattern along the unsusceptible direction. Consequently, it was indicated that the diameter distribution of the oscillating bubbles was correctly measured using the diffraction pattern along the unsusceptible axis by comparison with the reference value measured by the stroboscopic imaging method. (C) 2012 The Japan Society of Applied Physics