In order to investigate the friction and wear characteristics of surface modified spur gears made of stainless steel SUS440C in high vacuum, gear tests were carried out under non-lubricating condition in vacuum to be on the order of 10-4 Pa. In addition, dynamic load on spur gear teeth with surface modified layer was evaluated. Electroless Ni-P alloy plating (Ni-P plating), sulfurizing and Sn plating with thermal diffusion (Sn plating) were employed. From the gear tests, the following results were obtained. (1) The friction coefficient of the surface modified gears was smaller than that of the non-coated gears. (2) The initial friction of Sn plated gear increased under low pressure and low rotational speed conditions. After Sn plated layer wore uniformly, the friction coefficient was the smallest by the lubricating effect of thermally diffused layer of Sn. (3) The sulfurized layer contributed to the reduction of initial friction. However, the friction behavior of sulfurized gear was the same as that of non-coated gears, after the sulfurized layer wore completely. (4) Wear amount and tooth profile error of Ni-P plated gears were the largest of all the test gears.