Experimental studies are made of the fluid flow and mass transfer from an annular-finned circular cylinder in a crossflow. The naphthalene sublimation technique is employed to measure the local convective mass transfer rate around and along the circular cylinder between annular fins. Geometric parameter L/H, which is defined by the ratio of the distance (L) between annular fins to the height (H), was varied in the range 0<L/H ≤ 0.4. The influence of the horseshoe vortices on the local mass transfer rate was scrutinized depending on L/H at Red ~ 66,000. A remarkable enhancement of mass transfer due to the horseshoe vortices was found near the corner junction. The shapes of horseshoe vortices depend upon L/H. It is shown that the average mass transfer of overall surface at L/H=0.15 is twice larger than the average mass transfer without annular fins.