We report on the characteristics of a flexible Al2O3/Ag/Al2O3 multilayer passivation grown on a polyethylene terephthalate (PET) substrate as a function of Ag thickness. Due to the surface plasmon resonance (SPR) effects and the ductility of the Ag layer that is sandwiched between the dielectric Al2O3 layer, the flexible Al2O3/Ag/Al2O3 multilayer passivation exhibits a high transparency of 86.44% and improved flexibility at a Ag thickness of 10 nm. We found that SPR effects in the Ag layer occur at the transition region from distinct islands to a continuous film at a critical thickness (∼10 nm). In addition, the water vapor transmission rate of the Al2O3/Ag/Al2O3/PET sample decreased as the thickness of the Ag layer increased. Using synchrotron x-ray scattering and field emission scanning electron microscopy, we suggest a possible mechanism to explain the SPR in the Ag layer of the flexible and transparent Al2O3/Ag/Al2O3 multilayer passivation.