A simple 2D crystallization method using unfolded protein film as a supporting film of crystals was described, which allows modification of protein surfaces by injecting chemical reagents into the subphase after the crystal formation. As an example, glutaraldehyde was used to cross-link adjacent proteins and then stabilize protein crystals. The second layer of other proteins can also be formed on the apoferritin array using cross- linkers. The array of apoferritin is not only beneficial for electron crystallography but also for practical applications. For example, apoferritin produces a mineral core with a size which can be adjusted by the size to the cavity (i.e. 6 nm). Fabrication of such a small size of well defined fine particles is currently not easy using physical or chemical procedures. Using apoferritin, however, it is easy to produce uniform fine particles. If the core is designed to add interesting properties such as magnetism it is possible to make the highest class of magnetic film with ferritin 2D crystals. Basic researches toward practical applications of 2D protein crystal is now under way in various fields. The well defined size and function of protein molecules will benefit to many applications. The function and crystalline order can be designed by site-directed mutagenesis with the development of protein engineering.