We studied the gastric acid secretion and the histological findings of gastric mucosa in 48 patients with peptic ulcer before and after successful Helicobacter pylori (H. pylori) eradication. The pH of fasting gastric juice significantly decreased from 2.46 to 1.65 four weeks after eradication (p<0.001). The factors such as disease, atrophic border, inflammatory cell involvement and histological atrophy were analyzed by logistic regression. Only the atrophic border was a significant and independent factor to convert the acidity after eradication, and the pH of gastric juice prominently decreased in the patients with widespread atrophic mucosal area. In the 24-hour intragastric pH monitoring, the pH3 holding time overnight remarkably decreased from 54.1% to 22.3% after eradication (p<0.005). In the long follow-up of 17 patients (the mean follow-up period was 15.2 months), the pH of gastric juice kept a lower titer of 1.66. The mean atrophic scores in the upper body reduced from 1.5 to 0.9 (p<0.05). In addition to the functional recovery of parietal cells after eradication, it was considered that the histological improvement of atrophy in oxyntic mucosa was associated with the recovery of pH.