Abstract: A method that maximizes the minimum loop size in the routing-resource graph is proposed in FPGA routing resources architecture design to improve the flexibility of the routing resources.First analyze the relationship between the minimum loop size in the routing-resource graph and the transition flexibility of signals in the routing resources.Then maximize the minimum loop by adjusting the wires connections in the routing resources.Design a novel switch box,called minimum-loop maximization(MLM)switch box with this method.Experimental results show that while compared with 4 typical academic switch boxes,Disjoint,Universal,Wilton and JSB,MLM is at average level in timing and outperform all of them in routability by 17.7%,8.0%,2.4% and 2.2% separately.