Bacillus amyloliquefaciens α-amylase(BAA) and Bacillus licheniformis α-amylase (BLA) are useful enzymes in industry because of their thermostability. The sequence homology is 80% between BAA and BLA. The optimum temperature is 60-65℃ for BAA and 80-85℃ for BLA. To understand the difference of thermostability between BAA and BIA, the crystal structures of BAA and BLA have been determined by the molecular replacement method and the multiple isomorphous replacement method, and have been refined at 2.3Å and 2.2Å, respectively. The comparison of structure of BLA with that of BAA shows that the packing of side-chains inside the BLA molecule is more optimized than that within the BAA molecule.