In order to investigate frictional performances in different lubrication regimes and effects of surface roughness, the friction on engineering surfaces with different roughness patterns and amplitudes were measured. Results show a smooth transition of lubrication states from full-film hydrodynamic lubrication to mixed and boundary lubrication. Data from the tests also suggest that the transition of friction regimes is affected by roughness amplitude. It is observed that not only the smoother surfaces give rise to the lower critical velocities of transition from full-film to mixed lubrication, but also the friction coefficients at the point of the transition are much smaller for the smoother surfaces. For the transition from mixed to boundary lubrication, however, effects of roughness amplitude are insignificant. As a result, it is concluded that under the same contact conditions, different features in roughness cause the system to transit in different routines from one lubrication regime to another. Furthermore, a computer model, based on the deterministic solution of mixed lubrication combined with modelling of theological variation in ultra-thin films, has been presented to predict friction in overall lubrication regimes. At last, the comparisons of simulation and experimental results show satisfactory agreements.