Glycinamide ribonucleotide synthetase (GAR-syn, PurD) catalyses the second reaction of the purine biosynthetic pathway; the conversion of phosphoribosylamine, glycine and ATP to glycinamide ribonucleotide (GAR), ADP and Pi. In the present study, crystal structures of GAR-syn's from Thermus thermophilus, Geobacillus kaustophilus and Aquifex aeolicus were determined in apo forms. Crystal structures in ligand-bound forms were also determined for G. kaustophilus and A. aeolicus proteins. In general, overall structures of GAR-syn's are similar to each other. However, the orientations of the B domains are varied among GAR-syn's and the MD simulation suggested the mobility of the B domain. Furthermore, it was demonstrated that the B loop in the B domain fixes the position of the β- and γ- phosphate groups of the bound ATP. The structures of GAR-syn's and the bound ligands were compared with each other in detail, and structures of GAR-syn's with full ligands, as well as the possible reaction mechanism, were proposed.