A gene for a putative glucoamylase, stg, of a hyperthermophilic archae on Sulfolobus tokodaii was cloned and expressed in Escherichia coli. The recombinant glucoamylase (STGA) had an optimal temperature of $80^{circ}C$ and was extremely thermostable with a D-value of 17 hr. The pH optimum of the enzyme was 4.5. Being different from fungal glucoamylases, STGA hydrolyzed maltotriose (G3) most efficiently. Gel permeation chromatography and sedimentation equilibrium analytical ultracentrifugation analysis showed that the enzyme existed as a dimer. STGA was stable enough to hydrolyze liquefied com starch to glucose in 4 hr at $90^{circ}C$ with a yield of95%. Comparison of the $k_{cat}$ values for the hydrolysis and the reverse reaction at $75^{circ}C$ and $90^{circ}C$ indicated that glucose production by STGA was more efficient at $90^{circ}C$ than $75^{circ}C$. Therefore, STGA showed great potential for application to the industrial glucose production process due to its high thermostability.