In a nuclear power plant,when using a certain brand of lubricating oil for the main pump,the abnormal levels of Pb content appears in the lubricating oil in the case that no abnormal wear appears for the main pump.Through the laboratory simulation test and preliminary validation in the field,it was found that this brand lubricating oil had a strong effect on the separation out of metal Pb in the bearings,and the effect was proportional to temperature of operating oil.According to the test results,the main pump lubricating oil was replaced with another brand of oil which had a low ability of separation out of Pb.The field tests show that the Pb content in the lubricating oil has sharp decline.