Side weirs are kinds of hydraulic structures which are used in sewerage and irrigation and drainage systems. The flow over a side weir is a kind of spatially varied flow. Since there is no analytical solution for dynamic equation of spatially varied flow the empirical study is the best way for investigating these structures. Side weirs with a sharp crest have been extensively studied in the past but information on the characteristics of other side weirs is limited. In this study the hydraulic characteristics of circular crested side weir such as surface profile, velocity profile, discharge coefficient and separation zone have been investigated. Nineteen hydraulic models, divided in five groups were tested in a flume in the hydraulic laboratory of Isfahan University of Technology. Investigation of the surface profiles showed that the surface profiles in the vicinity of the side weir are nonuniform and this is due to separation zone close to the side weir. Therefore, the suitable place for measuring of the characteristics of the flow is along to the centre line of the channel. Investigation of the velocity profile showed that with increasing distance from the side weir in longitudinal and transection direction the effect of secondary flow was decreased and the velocity profiles became more uniform. Dimensional and statistical analysis indicated that discharge coefficient depend on upstream Froude number (Fr 1 ), the downstream slope of the circular crested side weir (?), the ratio of the weir height to upstream flow depth (P/Y 1 ) and ratio of weir length to upstream flow depth (L/Y 1 ).The result showed that the length of separation zone depends on upstream Froude number, the ratio of the weir height to upstream flow depth (P/Y 1 ), ratio of weir length to upstream flow depth (L/Y 1 ) and Reynolds number.