Abstract: Pull-off force of new cleavage mica contact pair was measured simultaneously and respectively in the variable pre-pressing force, separating velocity or relative humidity condition by surface forces apparatus, and the condition of the contact area was observed. Strain gauge was glued onto the cantilever spring, which had been calibrated accurately, at the fixed end, and strain indicator was used to record the strain-time curve. Pull-off force was calculated by contrasting the change of strain between minimal value and steady value to the calibration curve. The pre-pressing force and separating velocity were changed by controlling the feeding distance and velocity of lower sample. Within the elastic contact, the pre-pressing force make no difference on the pull-off force, the surface energy of contact material determines the pull-off force. When the pre-pressing force exceeds the critical point, plastic deformation happens and the pull-off force increases with the pre-pressing force; in dry contact condition, the pulloff force is independent of separating velocity, but when there is any liquid interface layer with large dynamic viscosity, it increase with the separating velocity in contrast; the relative humidity increases the pull-off force.