AIM: To analyze the clinical efficacy of intravitreal injection of ranibizumab (IVR) on diabetic macular edema (DME) with multifocal electroretinography (mfERG) and optical coherence tomography (OCT). METHODS: A total of 41 patients (41 eyes) with DME were treated with IVR. The best corrected visual acuity (BCVA), mfERG results, and OCT were analyzed to compare to the baselines at 1wk, 1, and 3mo after operation. RESULTS: The BCVA was significantly improved in all eyes at each time point (P<0.001). The macular area leakage and edema were reduced 1wk and 1mo after IVR, and the central fovea thickness (CFT) was significantly reduced compared to baseline (P<0.001). The mfERG, two-dimensional and three-dimensional images all showed that the macular fovea (1 ring) response density decreased, and the fovea and macular area spikes significantly decreased or disappeared. The amplitude density of the P1 wave was increased, and the latency of the P1 wave was shortened than preoperation (P<0.001). At 1wk and 1mo after the operation, there was a negative correlation between the amplitude density of P1 waves and CFT. CONCLUSION: OCT and mfERG fully demonstrate the importance of IVR for DME patients from the macular morphology and function, especially the significance of mfERG in this disease.