In the present study, we experimentally investigate the effect of tubercles on the hydrodynamic performance of a humpback whale flipper. The shape of the flipper used is the same as that of Miklosovic et al. (2004, 2007), and the Reynolds number considered is 100,000. The lift and drag forces on the flipper are measured by varying the angle of attack, and PIV measurements are conducted at a few different angles of attack. As observed in previous studies, with tubercles, the stall angle is delayed and the maximum lift coefficient is increased. Without tubercles, the cross flow above the flipper does not show large-scale vortical motions except tip vortex. With tubercles, however, strong streamwise vortices having negative vorticity are observed along the tubercles, but the vortices with positive vorticity are either relatively weak or unobserved. This result is very different from those found in a two-dimensional wing with tubercles with which strong counter-rotating streamwise vortex pair are observed.