The present study has been conducted in a 90 degrees sharp bend to investigate the effect of bank roughness on flow patterns. Tests conducted under two different flow conditions, with Froude numbers equal to 0.17 and 0.4 and two different roughness sizes of 0.5 and 5.0 mm. In each test three components of flow velocity at 576 point were measured. Plot of flow velocity vectors and stream lines were done by SURFER and TECPLOT softwares. The results show that the region of maximum flow at the beginning of bend occurs at the inner bank then gradually shifted toward the outer bank at the end of bend. Comparison of flow patterns in two different roughnesses shows that by increasing the roughness height, the strength of secondary flow decreases while the number of smaller rotational cells is increased. In general it may be said that in a sharp bend up to cross section of 70, the region of high shear stress is very closed to the inner bank and from that place to the downstream, it shifted to the outer bank. The increases of bank roughness causes that the maximum shear stress is decreases. The location of maximum and minimum shear stress at two roughness of 0.5 and 5.0 mm are at 20 and 90 respectively, their values have been calculated to be 1.47 and 0.54 N/m² for 0.5 mm roughness and 2.2 and 0.46 N/m² for 5.0 mm roughness, respectively.