A model is proposed to describe the effect of surface barrier (SB) on ac susceptibility (ACS) and a different kind of dip effect ~DE! in ACS is observed. Simulation based on this model with flux creep reveals two dips in ACS curve, one at temperature Td8 in real part x8 and the other at temperature Td9 in imaginary part x9. These two dips are different from the ones resulting from the peak effect in critical current density jc , where the dips in x8 and x9 occur at the same temperature Tp . The DE is also characterized by a large x9 and a large ux8(Td8)u. The ACS curves for single crystals Bi2Sr2CaCu2O8 have been observed and compared with the DE in YBa2Cu3O7 samples, confirming the numerical results. It is also shown that when flux creep is absent only kinks appear in x8 and x9 for a sample with SB.