A polyacetal gear of module 2 is engaged with a steel gear coated by gear oil or grease only initial stage, and different loads are applied to the set. Bending strengths and wears of polyacetal gear teeth under lubricated conditions are compared with those obtained under unlubricated conditions. Experiments are also carried out under the conditions of forced lubrication, and similar comparative studies are made on the results. Obtained test results are summarized as follows : (1) Reduction of bending strength of gear teeth caused by temperature increase is suppressed by lubricating gears. Furthermore, it is found that the extent of reduction of bending strength of gear teeth varied according to the lubrication methods applied even under the same load. The smallest is in the case of forced oil lubrication in which large amount of oil is used, followed by the case of grease coating, and the largest is in the case of oil coating. (2) Marked improvement of allowable loads, in which wears of gear teeth are taken into consideration, are recognized under the lubricated conditions compared to those obtained under unlubricated conditions.