Crystal structure of 3-isopropylmalate dehydrogenase from the moderate facultative thermophile Bacillus coagulans (BcIPMDH) has been determined by the x-ray method. BCIPMDH is a dimeric enzyme composed of two identicai subunits, each of which takes an open α/β structure with 11 α-helices and 14 β-strands. The polypeptide is folded into two domains. The first domain is composed of residues 1-101 and 257-356, and the second domain is 102-256. The latter domains are associated with one another by a dyad axis to form a dimer locally to form a β-sheet and a four-helix bundle. As compared with the structure of the enzyme from the extreme thermophile Thermus thermophilus (TtIPMDH), a new short P-sheet (residues 329-330 & 340-341) absent in TtIPMDH is formed by the insertion of 5 residues in BcIPMDH In the terms of determinants for thermostabilization, both consistent and inconsistent changes were found between the two enzymes. The regions including such inconsistent changes are formed by different usages of the determinants for stabilizing the loops at different levels. Those in BcIPMDH contain some structural redundancies in length of amino acid sequence and flexibility of residues, which seem to be unnecessary for the enzymatic reaction. Such redundancies are also found in the primary structure of the enzyme of the mesophile Bacillus subtilis, but these parts are more stabilized in BCIPMDH by hydrogen bonds and salt bridges. On the other hand, TtIPMDH is stabilized by reducing such redundant parts to adapt at higher temperature. This contrast suggests the two different strategies depending on temperatures.