Gas cluster ion beam (GCIB) irradiation on polyether-ether ketone (PEEK) surface have been carried out to enhance the cell adhesion. GCIB creates nanostructures with size of 10 - 50 nm on PEEK surface. These nanostructures are effective to create hydrophilic PEEK surface. By irradiation of O₂-GCIB on PEEK, increase of cell adhesion was observed. This cell adhesion enhancement was observed at oblique incidence, which indicates that GCIB irradiation is effective for three-dimensional structure.