We present a quantitative study of the frictional properties of self-assembled alkanethiol monolayers on Au-(111) as a function of chain length using scanning force microscope, The lateral and normal forces are calibrated in situ using a combination of the two-slope method proposed by Ogletree ct al, and the added mass method proposed by Cleveland er nl. The comparative study of the same system by another calibrated Si3N4 tip was carried out. The friction coefficients for the same alkanethiol system, but with different tips, differ by less than 15%, indicating the reliability of nanoscale frictional and normal force measurements using a scanning force microscope. An in situ experimental force calibration for each cantilever is needed in older to perform absolute force measurements. Our results also show that frictional fords depend strongly on the number of carbon atoms in the alkane chain. The friction coefficient of alkanethiol decreases as the chain length increases and is lower than that of a bare Au(lll) surface.