The effect of scan size (scan length) and surface roughness on microscale friction was studied using atomic force/friction force microscopy. Three silicon specimens with different surface roughnesses were studied. Surface height and friction force plots were obtained simultaneously and friction mechanisms for the correlation between them were sought. The standard deviation of surface heights increases with the scan size initially and approaches a constant value at a scan length greater than the long wavelength limit of the roughness structure. Change in the value of the coefficient of friction at different scan lengths is random. In these measurements, the sampling interval is always lower than the correlation length which is believed to be responsible for the random variation in microscale friction with the scan size. The good correlation observed between the local change in friction force and the surface slope can be explained by a ratchet mechanism. ©1997 American Institute of Physics.