The cantilevers of atomic force microscope (AFM) are mounted under a certain tilt angle, which is commonly assumed to have negligible effect on friction measurements in AFM. We present a theoretical study of the effect of the tilt angle on AFM based friction measurements. A method for correcting the friction coefficient between sample surfaces and AFM tips is also presented to minimize the effects of the tilt. The frictional forces between a silicon tip and a silicon surface at tilt angles ranging from 5 degrees to 25 degrees were measured. The results show that the measured friction coefficient increases with the tilt angle effectively, whereas the variation range of the corrected friction coefficient is within 10%.