Purpose: To evaluate the repeatability of macular hole (MH) diameter measurement on en face slab optical coherence tomography (OCT) reflectance images and assess its potential to predict visual acuity (VA). Methods: We enrolled 27 eyes with full-thickness MHs in this study. Preoperative en face slab OCT reflectance images were obtained. Image binarization, ellipse approximation, and uncorrected measurement of minimum diameter, min(ef_uc), and maximum diameter, max(ef_uc), were performed using ImageJ. In addition, magnification-corrected diameters were calculated as min(ef) and max(ef) using the Littman and modified Bennett formulas. Spectral-domain OCT horizontal images were used as the conventional method for the analysis: min(conv) and max(conv). The inter-rater reliability of the method was evaluated by calculating the intraclass correlation coefficient (ICC). The following relationships were analyzed: (1) between logMAR VA and min(ef_uc), min(ef), and min(conv); and (2) between logMAR VA and max(ef_uc), max(ef), and max(conv). Results: The min(ef) and max(ef) values were 439.4 +/- 240.5 mu m and 720.7 +/- 346.1 mu m, respectively. The ICC values were 0.985 and 0.999 for min(ef) and max(ef), and 0.885 and 0.909 for min(conv) and max(conv), respectively. Multivariate analysis suggested that min(ef), but not min(ef_uc) or min(conv), was associated with pre- and postoperative logMAR VA. Furthermore, max(ef), but not max(ef_uc) or max(conv), was also closely correlated with pre- and postoperative logMAR VA. Conclusions: The MH diameter measured by our method is highly reproducible and closely associated with VA compared to that measured by the conventional method. Translational Relevance: The MH diameter measured by this modality might serve as an accurate biomarker to predict visual function in eyes with MH.