Abstarct: Side weirs are among the important structures in water transmission networks. In oppose to simple side weirs, compound side weirs provide an accurate discharge measurements and are able to pass high rate of discharges. The rectangular-rectangular sharp-crested side weirs were studied experimentally and numerically in this research. The effective parameters on the discharge coefficient of a rectangular compound side weir were first determined using dimensional analysis. An empirical relation for discharge coefficient was then developed using least square of errors method. The predicted discharges using this empirical relation showed a very good agreement with measured discharges. The water surface profile of flow over the weir crest and in the center line of the main channel were then plotted and they showed that with increasing the Froude number of main flow the water depth over the weir crest and the weir discharge in turn increases. The water depth in the main channel also increased moderately. The Flow-3D software was employed to investigate the possibility of modeling the operation of compound side weirs. Four weir geometries were modeled in the software and the predicted discharges were compared with those of experiments. The results showed a good agreement approving the possibility of employing Flow-3D in determining the discharge of the compound rectangular side weirs. Finally the flow depth profile along the weir crest was drown which was very close to the experimental results.