The behavior of nanobearings constructed from double-walled carbon nanotubes (DWCNTs) is investigated with molecular dynamics simulations. The results show that the (5, 5)/(10, 10) DWCNTs can work as stable and reliable nanobearings to a speed as high as ∼2.65 r ps(-1) with an inner tube as rotator. When the speed is lower than ∼0.75 r ps(-1), the nanobearings remain in an ultrasmooth state, beyond which the intertube friction increases and fluctuates sharply. The rotational friction is sensitive to many factors such as rotation speed, radial size, and flexibility of CNTs. Increase in rotation speed and the radial sizes of CNTs leads to increase of centrifugal force and decrease of intertube distance, thus, increases the intertube friction. As a result, both the critical speed for ultrasmooth rotation and the ultimate speed decrease with increasing radius of the inner tube with constant intertube distance. The centrifugal force and thermal motion of atoms will stimulate flexile deformation of CNTs, namely waving tube axis and distorting cross-section, which will lead to an increase in rotational friction. When the outer tube serves as the rotator, the DWCNT nanobearing becomes more easily damaged.