The small heat shock protein (sHsp), categorized into a class of molecular chaperones, binds and stabilizes denatured proteins for the purpose of preventing aggregation. The sHsps undergo transition between different oligomeric states to control their nature. We have been studying the function of sHsp of Sulfolobus tokodaii, StHsp14.0. StHsp14.0 exists as 24meric oligomer, and exhibits oligomer dissociation and molecular chaperone activity over 80°C. We constructed and characterized StHsp14.0 mutants with replacement of the C-terminal IKI to WKW, IKF, FKI and FKF. All mutant complexes dissociated into dimers at 50°C. Among them, StHsp14.0FKF is almost completely dissociated, probably to dimers. All mutants protected citrate synthase (CS) from thermal aggregation at 50°C. But, the activity of StHsp14.0FKF was the lowest. Then, we examined the complexes of StHsp14.0 mutants with denatured CS by SAXS. StHsp14.0WKW protects denatured CS by forming the globular complexes of 24 subunits and a substrate. StHsp14.0FKF also formed similar complex but the number of subunits in the complex is a little smaller. These results suggest that the dimer itself exhibits low chaperone activity, and a partially dissociated oligomer of StHsp14.0 protects a denatured protein from interacting with other molecules by surrounding it.