To examine the effect of the difference of grease lubricating method on the scoring resistance of the stainless steel SUS 440C gears of which the surface layer was thermally diffused after being Sn-plated, scoring test was carried out by power-circulating gear test rig under forced lubrication, bath lubrication and spread lubrication. From the test, the following results were obtained. Under bath lubrication, the scoring resistance of the gears with the diffused layer was about 4.3 times as high as that of the gears without the diffused layer. Under forced lubrication, the scoring resistance of the gears with the diffused layer was about 2 times as high as that of the gears without it. Under spread lubrication, the destructive surface failure by scoring occurred on the meshing faces of the gears without the diffused layer after 20∼30 mins since the test run. But, for the case of the gears with the diffused layer, scoring did not occur while the test was running.