The confluence of two streams creates a complex flow which accelerates downstream. In a JUNCTION area, the separation zone is developed in the main channel. By decreasing the longitudinal velocity in the separation zone the sedimentation and blocking of section are increased. In this research, the separation zone dimensions for the JUNCTION 30o, 45o, 60o and 90o, and discharge ratios are simulated by using the FLUENT software. The Volume of Fluid (VOF) scheme and the RSM turbulence model were used for numerical simulation. The results showed that reducing the JUNCTION angle leads to decreasing separation zone dimensions at the JUNCTION in the main channel. Results also showed that the model simulated the experimental velocity values by an average of 4 percent accuracy. The maximum width of separation zone is about 50% of the main channel width occurred at an angle of 90o of JUNCTION channel, and the minimum dimensions of the flow separation zone near zero and occurred at the JUNCTION angle of 30o. Furthermore, increasing JUNCTION angle widens the separation zone dimensions. Maximum and minimum separation zone dimensions occurred at 90 and 30 degree of JUNCTION, respectively. Further changes in the discharge ratio have a great effect on separation zone dimensions and disappear when the discharge ratio is decreased. Decreasing JUNCTION angle and increasing the discharge ratio simultaneously, can reduce the separation zone dimensions and therefore, the problems developed the JUNCTION.