Background: Diabetes Mellitus has been a collection of metabolic diseases typified by persi-stently elevated blood sugar levels. Aim: This research used optical coherence tomography to examine the relationship among best corrected visual acuity & outer retinal layers thickness, which is the distance among the retinal pigment epithelium at the foveal center & the external limiting membrane. Methods: One hundred eyes of seventy-five studied cases had been comprised in this prospective, observational, case - control study. These cases had been collected from the ophthalmic out-patient clinic of Al-Azhar University Hospital, Assiut. These cases were split into two groups: Group 2 Diabetic Macular Edema (DME group) included fifty eyes from fifty diabetic studied cases, & Group 1 (Control group) included fifty eyes from twenty-five healthy individuals without a history of systemic or ocular illnesses. Results: There had been a highly significant correlation among BCVA & ORL (p < 0.001) & a strong correlation among best corrected visual acuity and central foveal thickness (BCVA & CFT) but less than the correlation among BCVA & ORL in DME studied cases. This research presented a statistically significant positive correlation among ORL thickness & CFT. Conclusion: The study shed light on the importance of outer retinal layers thickness as a novel OCT parameter in the evaluation of diabetic macular edema with a potential visual prognostic value. It also revealed thinning of both choroid and outer retinal layer (ORL) in DME cases which may be related to its pathogenesis.