The complexity of modern equipment is higher than before, thus it cannot meet the need of speedy fault diagnosis to use single diagnosis pattern. According to these troubles, a fault tree was introduced to analyze fault information of gear. Fault tree analysis is a logical and diagrammatic method to evaluate the probability of an accident resulting from sequences and combinations of faults and failure events. In conventional fault tree analysis, probabilities and consequences are treated as exact values. In many engineering applications, however, it is difficult to evaluate the probabilities and consequences from past experiences, so a fuzzy set defined in probability is proposed. The principle of fuzzy diagnosis is taken as basics and symptom sets and faults sets are extracted through fault tree analysis. Experiments show this method is effective for fault diagnosis of gear.