One of the effective ways for the flow of water in low head water conditions is using triangular duckbill weirs. Regarding the high efficiency of these weirs in higher rate of flow discharge in constant head water conditions, the investigation about this type of weirs is greatly required. The present study is done on triangular duckbill weir with two different vertex angle, installed in the laboratory straight channel. In the upstream of the installation place of the triangular duckbill weir, some flow guide vanes with different specifications are installed and the required tests were done in three different groups. The discharge coefficient was calculated by obtaining the required laboratory parameters, such as upstream water level and upstream velocity in free flow conditions. The ration of the flow discharge over triangular duckbill weir to the flow discharge of straight weir was calculated and the graphs relative to the total water head were drawn. The results show that the flow guide vane with smaller width has more effects on the efficiency of the triangular duckbill weir. Moreover, in all wall angle of located vane, the efficiency of weir with larger vertex angle is less than that with smaller vertex angle. Since the flow interference is increased by increasing the flow discharge in the triangular duckbill weir with low vertex angle, then, the presence of the flow guide vanes can reduce the horizontal component and increase the vertical component of upstream velocity over the weir. Thus, the flow passes with more velocity over the weir crest at the same time and the flow discharge rate is increased and therefore the efficiency of this weir is increased.
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