In order to investigate the optimum geometric condition for a heat transfer augmentation technique using fluid visco-elasticity, flow visualization experiments have been performed in a cavity between rectangular ribs mounted on one side wall of an open channel. Rib height and cavity size (the distance between ribs) channel flow with ribs on one side wall are changed in several steps while the rib length and the mean velocity at narrow cross-sections were kept constant. As viscoelastic fluid, dilute cationic surfactant solution of oleylbishydroxyethylmethylanmonium chloride was use. The rheological characteristics of the fluid were also measured for analyzing flow phenomena. From the results, the case having small rib height and large cavity size is found to have an advantage for minimizing a recirculating region formed in the downstream region. With the features of the first normal difference and apparent viscosity of the fluid, the size of the recirculating region is found to be proportional to the square value of the reciprocal of contraction ratio.