Nonlinear change of the refractive index (n) and extinction coefficient (k) of Co3O4 thin films induced by a laser with λ=405 nm (hν=3.06 eV) irradiation was evaluated using equipment having an ellipsometric optical configuration. Nonlinear refractive index (n2) and extinction coefficient (k2) were +1.0×10-10 m2/W (positive) and -2.6×10-11 m2/W (negative), respectively. n2 and k2 at λ=650 nm (hν=1.91 eV) were -5.5×10-11 m2/W and -8.7×10-11 m2/W (both have negative signs). From these results and the fact that the Co3O4 thin film has the band-gap energy of 2.06 eV, the band filling effect can be seen as one of the most probable models describing the large nonlinear change of n and k of Co3O4 thin film. © 2002 American Institute of Physics.