Contaminant is an important reason which causes the wind gear box failure.In order to study the effect of contaminant and wear particles on the gear box and guide the maintenance of gear box,the oil cleanliness level,moisture content and wear particle in the gear oils which were used in the 18 fans with normal operation for 4 years were tested.It is found that the oil cleanliness level in most gear oils cannot maintain in 17/15/11(ISO4406),the oil cleanliness level of some oil are even as high as 22/20/16(ISO4406).The particle size of most particles in the contaminant is from 4 to 6 μm,the maximum number of grains per milliliter is up to 24 911.The water content and acid value of the gear oils are all in the normal range which have little harmful to the oil and the gear box.In addition,a certain degree of wear occurs in the gears and bearings of the normal operating fan,which is mainly fatigue wear,and the particle of the largest size is caused by sliding wear.