Ventilated cavitating flow of a circular cylinder is investigated by numerical simulation. The coupled level set and volume of fluid method is used to capture the interface between the cavity and surrounding water. The simulation results indicate that the bubble size distribution in the wake is closely related to the turbulence intensity. The bubble number reaches its peak value in the closure region owing to the high turbulence intensity there. When the gas entrainment coefficient Q(v) increases, the length of the cavity increases and the turbulent kinetic energy decreases, leading to a decrease in the bubble number and an increase in the Sauter mean diameter. It is also found that the presence of the ventilated cavitation delays the formation of vortices and affects the vortex shedding. In the single phase flow, the enstrophy is concentrated in the shear layer and closure region. In the ventilated cavitating flow, on the other hand, the enstrophy is mainly distributed at the cavity interface and the re-entrant region, which indicates that the instability of the cavity interface and the existence of the re-entrant jet play important roles in the formation of vortices.