: Weirs are among the most popular and simple hydraulic structures. If the weir plate is placed verticaly to the bottom of the channel, and angled to the channel walls, is named an oblique weir. One easy and practical way to reduce the head for higher discharge with the limiting channel width and less energy loss, would be oblique weir.. Also, unlike full width weir, an oblique weir automatically aerated by diverting the flow through passing over the weir, hence, no aeration is needed to assure a free flow. There is a wide knowledge on perpendicular weirs,but few studies have been done on weirs placed obliquely to the flow direction (oblique weirs). In this study a comprehensive set of experiments is performed on weirs placed obliquely in an open channel. Since the previouse reaserchs have limitations on the angel of obliquness (from 0 to 64 degrees) and small ratios of H/P (H is the upstream water head and P is the weir height relative to upstream channel bed elevation), hence, the main objectives of this study is to investigat different wier obliqueness, different H/P, behavior and hydraulic characteristic of flow, and examining methods for increasing the discharge capacity and effective length of oblique weirs. Experiments were conducted on oblique weirs with different angles (from 0? to 80?), for both free and submerged flow. The effective crest length and threshold submergence (the first effective downstream water head above the weir crest to the upstream water head) are the other objectives of this study. Also different combinations of weir angle, guide vanes and upstream inclined plates were examined and the effects of each combination on increasing the discharge over the weir compared with the results of normal weir were determined. Results show that, the flow always tends to keep its direction to nearly perpendicular to the weir crest when it reaches and passes the weir. By increasing the oblique angle, the effective length of the weir increases significantly, whereas the discharge coefficient Cd slightly decreases. Finally, practical relations and design recommendations are presented on the discharge coefficient of oblique weirs, the effective length of oblique weir, threashold submergence and the use of guide vanes and inclined plates. It is seen that applying a group of 2 vane plates in upstream face of the weir under the water surface can increase the water dischrge upto 20% and by using the inclined plate the flow discharge can increase upto 25%. Similar analysis has been done on the effect of both devices on increasing the effective length of the weir. It is shown that guide vanes and inclined plate can increase the effective length upto 15% and 20% respectively.