This paper investigates the impact of inter basin water transfer from North Karun such as Koohrang number1&2 tunnels(under operation), Koohrang number 3 tunnel (under construction) and Behesht Abad tunnel (under Study), on the status of water resource in Behesht Abad and Koohrang basins using Water Evaluation and Panning System (WEAP) water balance model. The model was calibrated for a ten year period (1995-2004) and validated for a 30 years period respectively. .Results show that after the completion of all the proposed water resources development projects, river flows at both Behesht-Abad and Kohrang rivers will decrease by an average of 27 %. Also, considering the minimal environmental flow requirement for the downstream river, a total of 220 MMC of excess water can be transferred to the central plateau with a 95% confidence. By considering available regulated water resource, this figure can be increased to 470 MMC. Therefore, the maximum reservoir storage volume required is in Behesh-Abad dam is about 500 MMC. This is significantly lower than the suggested design storage of 1800 MMC; indicating that the proposed dam is over-designed by the consulting engineer.