Angle-resolved photoemission measurement has been performed on a twin-free Bi2Sr2CaCu2O8 single crystal to study the band-folding effect due to the BiO superstructure. It was found that the superstructure running along the b axis considerably modulates the electronic structure near the Fermi level (EF) in that direction, almost smearing out the dispersive feature of a band which crosses the Fermi level. In contrast with the energy region near EF, the main body of the valence band (1 to 7 eV in binding energy) shows a comparable but different energy dispersion between the a- and b-axis directions. This indicates that the band-folding effect may be different between the near-EF region and the whole valence band in Bi2Sr2CaCu2O8. © 1994 The American Physical Society.