In modern VLSI designs, flip-chip package is widely used to meet the higher integration density and the larger IO counts of circuits. Recently the IO buffers are mapped onto bump balls without changing the placement using re-distribution layer (RDL) in flip-chip designs. In this research, a sorting-based IO connection assignment for flip-chip designs is proposed to reduce the total wire length. The proposed method initially assigns the IO buffers to bump balls by sorting the Manhattan Distance between them, and then takes some pair-exchanges to modify the assignment. The experimental results show that compared with another partitioning-based IO assignment method, our proposed method can reduce the wire length by 12.94% on the average, at the expense of a little time consumption.