The thermostability of the recombinant α- and β-subunit homo-oligomers (α16mer and β16mer) and of natural chaperonins purified from cultured Thermococcus strain KS-1 cells was measured to understand the mechanism for the thermal acclimatization of T. KS-1. The β-subunit content of the natural chaperonin from cells grown at 90°C was higher than that at 80°C. The optimum temperature for ATPase activity of the natural chaperonins was 80–90°C, whereas that for α16mer and β16mer was 60°C and over 90°C respectively. Judging from the ATPase activity, β16mer was more thermostable than α16mer. The thermostabilities of the natural chaperonins were intermediate between α16mer and β16mer, whereas the natural chaperonin with a higher β-subunit content was more stable than that with a lower β-subunit content. Native polyacrylamide gel electrophoresis (PAGE) revealed that the chaperonin oligomers thermally dissociated to their ATPase-inactive monomers. The thermal denaturation process monitored by circular dichroism showed that the free β-subunit was more stable than the free α-subunit, and that the secondary structure of the chaperonin monomer in the oligomer was more stable than that in the free monomer. These results suggest that the structure of these subunits was stabilized in the oligomer, and that an increase in the β-subunit content conferred higher thermostability to the natural hetero-oligomeric chaperonin.