The MO spectra of GMR structures with capping layers, Si/SiO2/bottom electrode/TbFeCo (9 nm)/Cu (4 nm)/ Co (0.1 nm)/Pt (1.1 nm)/(Pt (1.1 nm)/Co (6 nm)!4)/cap (cap = Ta, Au, Pt (3 nm)), were studied. We found that the optical constants and Kerr spectra exhibited different behaviors for all samples. The differences in the optical constants of these samples were analyzed by means of optical simulation. The MO spectrum is discussed in terms of simulation carried out by the virtual optical constant method for (PtCo)/cap (cap = Ta, Au, Pt (3 nm)).