Side weir is used as a slot in the side wall of the main channel to direct excess water that is above the weir crest. Such structures are used in control of flow level in irrigation, drainage networks and urban sewage disposal systems. Furthermore, circular channels are pretty important because considerable length of sewage disposal and transmission pipeline system is made of circular channels. In this study, changes inflow free surface, turbulence and flow field passing through a circular channel along a side weir are simulated. This means that for modeling the variations in flow free surface and the flow field turbulence, the volume-of-fluid scheme and RNG k-ε turbulence model are used, respectively. In order to validate the accuracy of the numerical model, the flow free surface changes along the side weir, the discharge coefficient of side weir, discharge of side weir, Froude number at upstream of the side weir and specific energy are compared with the experimental measurements. Comparison of simulation results with the experimental measurements shows high accuracy of CFD model. In other words, the root-mean-square error percent and the mean absolute error for flow free surface are, respectively, obtained to be 0.554% and 0.547%. Moreover, average difference between the specific energy at upstream and downstream of the side weir is calculated about 2.1%. The main purpose of this simulation is gaining an understanding of behavior of the passing flow through circular channels with side weir for subcritical flow regime.