To investigate the efficacy of early surgical intervention in ameliorating aniseikonia among patients with epiretinal membrane. This prospective cohort study enrolled patients who underwent surgery for epiretinal membrane. Patients were divided into early (symptom onset within 1 year) and late (symptom onset ≥1 year) treatment groups. Changes in aniseikonia, best-corrected visual acuity, and tangential retinal displacement were assessed and compared at 6 and 12 months postoperatively. Of the 56 patients, 30 (53.6%) belonged to the early treatment group and 26 (46.4%) to the late treatment group. The early treatment group demonstrated a significant reduction in aniseikonia score at 6- and 12-month follow-up visits (-1.10 ± 1.50 [P = 0.002] and -1.18 ± 1.79 [P = 0.003], respectively); however, no improvement was observed in the late treatment group (0.98 ± 4.62 [P = 0.310] and 1.52 ± 4.35 [P = 0.124], respectively). The early treatment group showed larger tangential retinal displacement at the 12-month postoperative follow-up visit. In addition, the amount of tangential retinal displacement was associated with postoperative changes in aniseikonia. Early surgical intervention is helpful in improving aniseikonia in patients with epiretinal membrane. The degree of recovery in inner retinal displacement was associated with the improvement of aniseikonia.