Archaea or archaebacteria are the microorganism living in extreme environments such as hot springs and salt lakes. The membrane is featured universally by lipids which possess saturated polyisoprenoid chains in the hydrophobic moiety. This paper concerns the surface properties of Langmuir membranes made of archaeal lipid models (AL) bearing a phytanyl group or (3RS, 7R, 11R)-3,7,11,15-tetramethylhexadecyl group. All of the AL provide a Langmuir membrane on an air-water interface with an abnormally low surface tension (32-37 mN/m at 20-70 degrees C), while the conventional lipids having n-alkyl chains give membranes of 54-56 mN/m. The abnormally low energy surface of AL lipids is considered to arise from the bulky and fluid polyisoprenoid chain.