Absorption and luminescence saturation of confined excitons are measured using a single‐beam method for CuCl microcrystals with various radii embedded in NaCl single‐crystalline matrices. From the excitation‐intensity and microcrystal‐size dependencies of their saturation, the effective saturation density of excitons, which is inversely proportional to the magnitude of optical nonlinearity, proves to be inversely proportional to the volume of microcrystals with effective radii of less than 5 nm at 77 K. Therefore, the optical nonlinearity of these microcrystals is not a function of microcrystal radii, but of the number of excitons created in a microcrystal, indicating the characteristic nature of coherent exciton confinement. ©1996 American Institute of Physics.