Background/Aims: The long-term effect of Helicobacter pylori eradication on the metabolic syndrome or diabetes are unclear. The aim of this study was to evaluate the effect of H. pylori eradication on glycemic control in type 2 diabetes mellitus (T2DM) or prediabetes mellitus (preDM). Methods: A total of 124 asymptomatic subjects with T2DM or preDM were divided into H. pylori-negative (n = 40), H. pylori-positive with non-eradicated (n = 34), and eradicated (n = 50) groups. We measured H. pylori status (culture, histology, and rapid urease test) and glycated hemoglobin A1c (A1C) levels and followed-up at the 1st year and the 5th year of follow-up. Results: The A1C levels significantly decreased in the eradicated group compared to the negative group and the non-eradicated groups (at the 1st year, p = 0.024; at the 5th year, p = 0.009). The A1C levels decreased in male, and/or subjects < 65 years of age in subgroup analyses (in male subjects, p = 0.047 and p = 0.020 at the 1st and the 5th year; in subjects < 65 years of age, p = 0.028 and p = 0.006 at the 1st and the 5th year; in male subjects < 65 years of age, p = 0.039 and p = 0.032 at the 1st and the 5th year). The eradication of H. pylori was related to the decrease in A1C values throughout the follow-up period, compared to the non-eradicated group (p = 0.017). Conclusions H. pylori eradication was related to the decreasing of A1C levels in patients with T2DM or preDM over a long-term follow-up period, especially in male and subjects < 65 years of age.