In 1977, third domain of Life gArchaeah was proposed by Woese et al, based on phylogenetic analysis of 16s rRNA sequence. Archaea are single-cell organisms that have spheric and rod shape morphologies and are not distinguishable from Bacteria in cell shape and size. Although Archaea are prokaryotes, it is more similar to Eukarya than Bacteria in biochemical characters. Archaea have the unique cell membrane made of ether lipid. The domain includes thermophiles (isolated from hot springs and deep sea hydrothermal areas), extreme halophiles (found in salt fields and highly concentrated salt lakes) and methanogens (living in anaerobic marsh and in animal intestin). Research of Archaea will contribute to extend the knowledge on common ancestor and the origin of Eukarya.