Abstract: Five Bacillus coagulans strains were studied for lactic acid production,and based on the genome of B. coagulans ATCC7050,the key lactic dehydrogenase gene in lactic acid production was determined. The results showed that,there are two types of lactic acid produced by Bacillus coagulans:L-type lactic acid and D-type lactic acid,but mainly L-type lactic acid,and its optical purity was more than 97%. In the genome of Bacillus coagulans ATCC7050,there were three lactate dehydrogenase genes:ldhL1,ldhL2 and ldhD. The transcriptional signal was not detected in the ldhL2 gene,while the transcriptional level of the ldhL1 gene in the logarithmic phase was 68 times as that of the ldhD gene,which indicated that the ldhL1 gene was the key gene for lactic acid production by ATCC7050. So the most important lactic dehydrogenase gene was identified in the lactic acid synthesis in ATCC7050,and it laid the foundation for metabolic research and genetic engineering of B. coagulans.