Certain enzymes utilize the high reactivity of radicals to catalyze chemically difficult reactions. Coenzyme B_<12> serves as a cofactor for enzymatic radical reactions. The three-dimensional structures of coenzyme B_<12>-dependent diol dehydratase and glycerol dehydratase were determined by X-ray crystallography. The structure-based fine mechanism of action of diol dehydratase was studied to establish the general mechanism for B_<12> enzymes as well as radical enzymes. The steric strain model was proposed for the coenzyme cobalt-carbon bond homolysis. The ribosyl rotation model well explained the distance problem and the stereospecificity in hydrogen abstraction. The substrate-induced conformational change of the enzyme revealed the substrate triggering mechanism for the catalytic radical formation. Theoretical calculations as well as mutational studies suggested that the hydroxyl group migrates by the concerted pathway through a three-membered cyclic transition state which is stabilized by active-site amino acid residues. A refined catalytic mechanism for diol dehydratase is proposed here.