To evaluate prognostic factors for metamorphopsia in patients undergoing vitrectomy for epiretinal membrane using spectral domain optical coherence tomography. This study included 53 eyes of 53 patients undergoing vitrectomy for idiopathic epiretinal membrane. The authors examined visual acuity, severity of metamorphopsia with M-CHARTS, and retinal microstructures with spectral domain optical coherence tomography before surgery and 3 months and 6 months after surgery. Central foveal thickness, parafoveal retinal thickness, macular volume, thickness of the ganglion cell layer, inner nuclear layer, and outer retinal layer (ONL + OPL: outer nuclear layer and outer plexiform layer) were analyzed using an image processing program. The status of the outer retinal lines was also evaluated. Preoperative metamorphopsia score (M-score) significantly correlated with central foveal thickness, inner nuclear layer thickness, and ONL + OPL thickness. Postoperative M-score significantly correlated with postoperative central foveal thickness and inner nuclear layer thickness. Inner nuclear layer thickness had significant correlation with M-score at each time point by multiple regression analysis, whereas other parameters were not relevant. Multiple regression analysis showed that preoperative inner nuclear layer thickness yielded the highest regression coefficient with postoperative M-score. Inner nuclear layer thickness was found to be a good indicator of metamorphopsia both before and after surgery and a predictor of postoperative metamorphopsia in patients with epiretinal membrane.