AIM: To examine the change of iris volume measured by CASIA2 anterior segment optical coherence tomography (AS-OCT) in glaucoma patients with or without type 2 diabetes mellitus (T2DM) and explore if there is a correlation between hemoglobin A1c (HbA1c) level and iris volume. METHODS: In a cross-sectional study, 72 patients (115 eyes) were divided into two groups: primary open angle glaucoma (POAG) group (55 eyes) and primary angle-closure glaucoma (PACG) group (60 eyes). Patients in each group were separately classified into patients with or without T2DM. Iris volume and glycosylated HbA1c level were measured and analyzed. RESULTS: In the PACG group, diabetic patients’ iris volume was significantly lower than those of non-diabetics (P=0.02), and there was a significant correlation between iris volume and HbA1c level in the PACG group (r=-0.26, P=0.04). However, diabetic POAG patients’ iris volume was noticeably higher than those of non-diabetics (P=0.01), and there was a significant correlation between HbA1c level and iris volume (r=0.32, P=0.02). CONCLUSION: Diabetes mellitus impact iris volume size, as seen by increased iris volume in the POAG group and decreased iris volume in the PACG group. In addition, iris volume is significantly correlated with HbA1c level in glaucoma patients. These findings imply that T2DM may compromise iris ultrastructure in glaucoma patients.