This chapter describes the purification and crystallization of oxygen-evolving photosystem II core dimer complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus . Procedures used for purification of photosystem II from the cyanobacterium involves cultivation of cells, isolation of thylakoid membranes, purification of crude and pure photosystem II core complexes by detergent solubilization, followed by differential centrifugation and column chromatography. The purified core dimer particles were successfully used for crystallization, and the methods and conditions used for crystallization are presented. These purification and crystallization procedures can be applied for another thermophilic cyanobacterium T. elongatus .