Abstract: Mj HSP16.5 is a small heat shock protein (sHSP) from the hyperthermophilic methanoarchaeon, Methanococcus jannaschii (Mj), which lives at the environment of high temperature up to 94 ℃. The structural data showed that Mj HSP16.5 was a 24-mer that formed a hollow sphere with octahedral symmetry. Mj HSP16.5 was very stable at pH 7 that it maintained the 24-mer structure even at 85 ℃. In the present study, we investigated the unfolding process of Mj HSP16.5 in the presence of denaturants using several techniques, including circular dichroism (CD), dynamic light scattering (DLS), fluorescence spectroscopy, and size exclusive chromatography (SEC). We found that 8 mol·L-1 urea had no obvious effect on the structure of Mj HSP16.5 at pH 7. The unfolding of Mj HSP16.5 at pH 7 in the presence of guanidine hydrochloride (GdHCl) showed hierarchical behavior. Three significant transitions were observed around 2.0, 3.0, and 6.0 mol·L-1 GdHCl at pH 7. ANS (8-anilino-1-naphthalenesulfonic acid) titration results showed that the binding ability of Mj HSP16.5 to ANS decreased gradually as the concentration of GdHCl increased until around 2.0 mol·L-1 GdHCl, indicating surface hydrophobic area change, and this first transition was companioned with precipitation of Mj HSP16.5. Acrylamide quenching of fluorescence showed that the Stern-Volmer constant changed at about 3.0 mol·L -1 GdHCl, indicating changes of the dimeric interface, and this phase transition was companioned with oli meric state change from 24-mer to small oli mers (4-mer to 8-mer). The last unfolding phase started around 5.0 mol·L-1GdHCl, with a midpoint of 6.1 mol·L-1 GdHCl, and Mj HSP16.5 was completely unfolded at 7.0 mol·L -1 GdHCl. We also found that Mj HSP16.5 could be quite easily unfolded at pH 3, where it could be completely unfolded in 4.0 mol·L-1 GdHCl.