The gene encoding a cystathionine γ-synthase from Sulfolobus tokodaii was cloned and expressed in Escherihia coli Rosetta-gami (DE3). Cystathionine γ-synthase ［EC 2. 5. 1. 48］ from Sulfolobus tokodaii (stCGS) was purified by heat treatment, DEAE- Toyopearl 650M and Sephacryl S-300 column chromatographies from E. coli transformants. stCGS shows optimum activity at pH 7.0, and is stable between pH5.0 and pH9.0. The optimum temperature of stCGS is above 100℃, and the enzyme showed the remaining activity of almost 100% up to 70℃. The K(m) and V(max) with O-phospho-L- homoserine as a substrate are 0.82 mM and 2.42 U/mg. To analyze the role of Phe 97 in the active site of stCGS, we constructed F97Y, R99C, and F97Y-R99C mutant enzymes. Although native stCGS has no activity toward l-methionine, F97Y mutant enzyme gained the elimination activity toward L-methionine.