The lube oil separator is widely used in lube oil system of merchant vessel. The lube oil separator functions to purify the lube oil to ensure its good working performance. The working condition of the oil separator will greatly influence the normal running of many plants. The lube oil separator used in our laboratory is Mitsubishi SJ700. In the processing of purifying, oil leakage from the water outlet is found. In order to find failure point, the basic structure and the working principle of the separator is introduced. The separator is dismantled according to the procedure. After cleaning some parts found dirty and replacing some sealing elements, the separator is reassembled. When starting the separating process again, the failure remains. Through further analyzing the structure and working principle of the separator, the trouble spot is found out. After removing the failure, the separator finally is repaired. To prevent similar accident occurring again, some suggestions are presented.