Abstract: An interconnected power grid makes full use of power resources in the broader power grids via tie-line power transmission. Exact description of tie-line transfer capacity region guarantees the optimality and security in the dispatch. This paper proposed a new method for exactly determining the tie-line transfer capacity region of interconnected power grids via multi-parametric programming, with a special focus on DC tie lines. The proposed method selects the DC tie-line powers as the parameter. An exact DC tie-line transfer capacity region is found, which satisfies operational constraints, including power balance constraint, generator capacity constraint and branch flow constraint. The simulation results of the joint case IEEE 9-30-bus and 14-39-bus test system demonstrate the validity and effectiveness of the proposed method. In terms of accommodating high renewable penetration across the interconnected internetwork, the impact of renewable penetration is further analyzed for the tie-line transfer capacity region. This quantitative analysis provides the decision-making basis for the rational allocation of renewable energy.