AIM: lo investigate the mutation in D-loop region of mitochondrial DNA in gastric cancer and its influence on the changes of reactive oxygen species (ROS) and cell cycle. METHODS: The D-loop region was amplified by PCR and sequenced. Reactive oxygen species and cell cycle were detected by flow cytometry in 20 specimens from gastriccancer and adjacent normal tissues. According to the sequence results, gastric cancer tissue was divided into mutation group and control group. Reactive oxygen species, apoptosis and proliferation in the two groups were compared.RESULTS: Among the 20 gastric cancer specimens, 18 mutations were identified in 7 patients, the mutation rate being 35%. There were four microsatellite instabilities in the mutations. No mutation was found in the adjacent tissues. Reactive oxygen species, apoptosis, and proliferation in the mutation group were all significantly higher than those in control group.CONCLUSION: Mutation in D-loop region plays a role in the genesis and development of gastric cancer.