The gearing motion of worm gears is characterized by sliding. To obtain worm gears of high performance, therfore, it is essential to make the actual contact area large enough by finishing them precisely, and exercise special care in the design, manufacture and assembling, in order that the lubricant may pervade evenly all over the contact area. The authors tested the performance of actual worm gears by means of the Power circulating Worm gear testing machine and made it clear that the entry gap and the running-in were very important. The authors also clarified that worm gears were in action under the condition of boundary lubrication.