Inosine monophosphate dehydrogenase(IMPDH, Ec126.96.36.199) is the rate-limiting enzyme for de nove guanosine monophosphate synthesis. The IMPDH encoding gene guaB has been clonded and sequenced from Bacillus amyloliquefaciens GR600, a overproduction-guanosine strain. A fragment contained the stuctrural gene guaB encoding IMPDH from GR600 was constructed into expression vector pET-His. The recombinant expression plamid was transformed into Escherichia coil strain BL21(DE3), induced by IPTG and expressed. The recombinat IMPDH was purified by Ni-NTA resins. The result of SDS-PAGE showed that molecular weight of the recombinat IMPDH was 54 kD. Enzyme activity assay showned that the optimum pH value and temperatrure of the recombinat IMPDH were 8.0 and 40°C. The results have great significance in genetical modify of producing strain.