Abstract: A HMn58-2 copper cooler cracked after being stored in the ware house for four years. The cracking failure cooler was investigated by optical microscope, mechanical testing machine and brinell hardness tester. The results show that coarse grain of heat-affected zone, produced by improper welding procedure or improper welding operation and formation of Widmanstaten microstructure by precipitated of α-Cu phase from grain boundary of β phase decreased the ductility, plasticity and strength of material of the cooler. In addition, the stress exited in copper plate was large because it was not anneal in time after cold coiling. So cracks initiated at the welding bead and extended continuously during the storing process and eventually led to cracking of the copper cooler.