Abstract: Cavitation is one of the mechanisms that lead to tissue erosion by high intensity focused ultrasound (HIFU). High-speed photography was used to study spatial distribution characteristics of cavitation bubble cloud structure in HIFU field, the influence of ultrasonic power to the structure, and its formation process with special attention to its inception process. The structure of the cavitation bubble cloud is featured by the regional distribution of cavitation bubbles. Within a range of ultrasonic power levels the structure remains stable. In terms of its formation process, cavitation bubble cloud first emerges in the focal region, then appears in the post-focal region (far from the transducer) and finally forms in the pre-focal region (near to the transducer). During the inception process of the cavitation bubble cloud, the initial photographed bubble cloud is approximately the same shape as the focal region, and cavitation bubbles are observed to coalesce and collapse in the focal region.