flow in natural river bends is a complex and turbulent phenomenon which affects the scour and sedimentations and causes an irregular bed topography on the bed. for the reason, the flow hydralics and the parameters which affect the flow to be studied and understand. in this study the effect of bed and wall roughness using the software fluent discussed in a sharp 90-degree flume bend with 40.3cm wide and ratio of radius to width equal to 1.5. in this study four different roughness of 0.5, 2, 5 and 10 mm were studied under flow discharges of 13.55 lit/sec, the flow depth in all studies were kept constant equal to 9 cm. laboratory study had been done previously on an experimental model made up of compacted plastic materials in ferdowsi university of mashhad. after comparing the velocity and water depth result with experimental tests and gaining confidence about fluent function, the effect of roughness on longitudinal velocity, depth water and secondary flow was investigated using software fluent. the results showed that by increasing the roughness height from 5 mm to 10 mm in all the sections water depth is less than 3% compared to the smooth channel mode will increase. the maximum longitudinal velocity approaches the internal bank from one time as much as width before the bend, and it approaches the external bank about 45 degree. roughness has had a considerable effect on the reduction longitudinal velocity. this decrease of velocity is greater with increasing roughness so that from between the roughness of 0.5, 2, 5 and 10 mm, roughness height 10 mm has the greatest effect on reducing the rate of velocity. in sections of 45 and 90 degree of bend, near the edge of the outer bend and near surface of water observed a circle clockwise secondary flow that by increasing the roughness height this secondary circle has been eliminated approximately.