The method of Cartesian product is widely used as constructing large interconnection networks from many specific small networks.In this paper,we study the edge fault-tolerant diameter of Cartesian product graphs,which is an important measurement for reliability and efficiency of interconnection entworks.Let G1,G2 be t1-edge-connected graph and t2-edge-connected graph resprctively,then the edge fault-tolerant diameter of G1×G2 has an optimal upper bound that D't1+t2（G1×G2）≤D't1(G1)+D't2(G2)+1,where t1,t2≥1 .