Abstract: The nozzle tube of engine oil cooling nozzle of a diesel engine fractured at the copper brazing seam in the course of service. The fracture reason was analyzed by the methods such as microscopic morphology observation, chemical composition analysis, microstructure observation and hardness test, and the quality control measures were proposed. The results show that the molten copper penetrates the austenite grain boundaries in the brazing process and brittle film is formed, which weakens grain boundaries’ cohesive force. This type of cracking is known as copper-contamination cracking. The crack propagates from copper-contamination cracking zone and leads to the fracture of nozzle tube when it is subjected to external force.