Amino acid dehydrogenases are oxidoreductases catalyzing the reversible deamination of amino acids to the α-keto acids in the presence of NAD or NADP. These enzymes play an important role at the junction between amino acid and organic acid metabolism, or organic and inorganic nitrogen metabolism in living cells. In this study, NAD-dependent leucine and alanine dehydrogenases from Bacillus sphaericus were purified and crystallized, and their physicochemical and enzymatical characteristics and antitumor activity were demonstrated. The peculiarities of the thermostable leucine dehydrogenase purified from B. stearothermophilus were also presented. The thermostable leucine and alanine dehydrogenase genes were cloned in Escherichia coli for high production of these enzymes. Simple and rapid purification methods of the two enzymes from the cloned cells are elucidated. Moreover, some applications of these enzymes were demonstrated : e.g., the determination of amino acids and α-keto acids, clinical analysis and production of amino acids in an enzyme membrane reactor.