The variable angle spectroscopic ellipsometer in conjunction with computer simulation were employed to determine the optical constants of ZnO in the ZnO/Ag/ZnO (ZAZ) multilayer system. A five-phase, glass/ZnO/Ag/ZnO/air, model was used to fit the calculated data to the experimental spectra. The ellipsometrically determined ZnO and Ag layers thicknesses were found to be agreed well with those previously determined by X-ray reflectivity. The effects of Ag layer thickness (Agd) and ZnO top layer thickness (ZnOd) on the optical constants of ZnO in the ZAZ multilayer system were discussed. The refractive index values determined at 550 nm, remains almost constant as Agd increases while the refractive index values decrease with decreasing ZnOd. Over the whole visible spectral range the extinction coefficient values of the Agd group are very close to zero and does not depend on Agd while the extinction coefficient values of the ZnOd group increase with decreasing ZnOd. The optical band gap values are found to be affected strongly by both Agd and ZnOd.