Plasma-surface modification is a technique to treat surfaces of various materials for a wide variety of applications. However, the treated surface layer is so thin that the nature of the treated surface decays gradually with time. The reasons for the decay of plasma-treated surfaces are considered to be (1) rotation and migration of surface moieties containing functional groups into inside of the material and (2) disappearance of relatively low-molecular-weight substances including weak boundary layer (WBL) formed in the plasma treatment. However, the decay behavior depends on materials nature, plasma condition, storage condition, and so on, and the precise mechanism is still unknown. The decay is also observed on inorganic material surfaces. Promising methods for the prevention of the decay, at the present stage, may be surface cross-linking and immobilization or grafting of suitable polymers on the surface.