Using the first-principles method, the absolute band edge positions were calculated for the (110) surface of zinc-blende type II–VI semiconductors. The thickness of the slab which was used to model the (110) surface has significant effect on the band edge positions. The variation trend of E g with the slab thickness is dominated by that of E CBM , while almost irrespective of E VBM . This phenomenon can be interpreted by the electron (at CBM) and hole (at VBM) effective masses. The absolute band edge alignments with different slab thickness can give reasonable interpretation for a series of photocatalytic experiments.