Abstract: Test data of cavitation on blade surfaces were collected from a model bulb turbine running in on-cam operating conditions under characteristic working heads, with focus on generation and development of the cavitation. Gap cavitation was also observed in the model tests and all the observations for the whole operating range were recorded in photographs and sketches. This study shows that, the critical cavitation coefficient corresponds to a flow condition characterized by a sharp change in turbine efficiency for a fourth η-σ curve. And along with the decreasing cavitation number, cavitation on blade surfaces or in the gaps at runner tips was developing though four stages: travelling cavitation bubbles, sheet cavitation bubbles, cloud cavitation bubbles, and super cavitation bubbles. Locations of cavitation erosion can be predicted according to cavitation characteristics. Higher precision of the discharge ring and blade flange or a more reasonable flange clearance can suppress gap cavitation.