The aim of the study is to observe the effect of outer hexagonal perforated cylinder surrounding an inner cylinder on vortex structure behind bluff body. Vortex shedding characteristics from bluff body are experimentally studied by particle image velocimetry (PIV) in deep water. The inner cylinder had seven different diameters (D-i), varied from 30 to 90 mm with an increment of 10 mm and the hexagonally perforated outer cylinder placed outside of the inner cylinder was used as a control element. Experiments were carried out with six perforated cylinders with a diameters of 100 mm (D-o) and six porosities, beta, with hole diameter 10 mm. The porosity, beta, and diameter ratio, D-i/D-o, had substantial effect on the flow characteristics behind the cylinder. For the range of diameter ratios of 0.3 <= D-i/D-o <= 0.5 the optimum porosity that controls the flow structure is found to be beta = 0.6. For 0.6 <= D-i/D-o <= 0.7 it is beta = 0.5 and beta = 0.4 for 0.8 <= D-i/D-o <= 0.9. With increasing diameter ratio, D-i/D-o, peak magnitude locations of Reynolds shear stress, < u'v'/U-2 >, and turbulence kinetic energy approach the inner outer cylinder system slightly. For all diameter ratios, beta = 0.6 is the most effective porosity ratio.