Objective To evaluate the roles of folic acid and β-carotene in the chemoprevention of gastric and other gastrointestinal (GI) cancers. Results A total of 7 new cases of gastrointestinal cancers were diagnosed with 3 stomach, 1 colon and 1 esophageal cancers occurring in the placebo group; 1 stomach cancer in both of the N-βC and S-βC groups, and no cancer occurring in FA group. In terms of GI cancers, there was a significant reduction in the FA group, compared with the placebo group (P=0.04). A similar trend was observed in both N-βC and S-βC groups (P=0.07-0.08). Taken together, the three intervention groups displayed a highly significant decrease in occurrence (P=0.004, vs placebo), and a lower risk for GI cancers (OR=0.12; 95% confidence interval, 0.03-0.51). For development of gastric cancer, any one of the three active-treated groups did not reach statistically significant reduction. The FA group showed obvious improvement of the gastric mucosal lesions with more patients displaying lesions reversed or stable atrophy and inflammation (P=0.04), reversed intestinal metaplasia (P=0.06) at the end of follow-up, and reversed displasia (P=0.017) at 12 months. Two cases of false jaundice were found in β-carotene groups with no influence on administration, and no side-effects were reported in FA group. Conclusions This trial revealed the interventional effect of folic acid on the development of GI cancers, a similar effect of β-carotene was also detected. Also, folic acid may be of use to treat atrophic gastritis by preventing or reversing the precancerous lesions.