A naphthalene sublimation technique is employed to investigate the three-dimensional secondary flow structure and the characteristics of convective mass transfer around a circular cylinder with annular fins. Mass transfer measurement is performed in the overall surface between annular fins with a different L/H, where L is the length between annular fins and H is the height of annular fin from a circular cylinder surface. A three-dimensional positioning apparatus is designed to make the accurate surface-profile measurement. Distributions of local circumferential Sherwood number show high value on the front stagnation point and a separation bubble is found in the cases less than L/H=0.15. For a circular cylinder with annular fins, the high augmentation of mass transfer rate is found near L/H=0.15. The surface plots and contours of mass transfer rates give the predictions of the flow structure. A remarkable enchancement of mass transfer due to the horseshoe vortex system is observed near the annular fin-cylinder junction. The local Sherwood number in the spannwise direction represents the secondary flow V2 and the corner vortex V3 near the wall of fin. The average Sherwood number of overall surface around L/H=0.10-0.15 is shown 1.5-2.0 times larger than that of without annualr fins. The correlations of total average mass transfer rate varies with L/H and $Re_L$.
