Background: Optical coherence tomography (OCT) is a non-invasive instrument, which can be used to estimate the thickness of the retinal nerve fibre layer (RNFL) and provides an indirect measurement of axonal destruction in multiple sclerosis (MS). The main aim of this study was to find out any correlations between P100 latency in visual evoked potential (VEP) and RNFL thickness.Methods: The patients with the definite history of optic neuritis regardless of the diagnosis of MS were included. The eyes with the history of blurred vision and increased VEP latency (> 115 milliseconds) were considered as cases and the eyes with normal latency were regarded as controls. RNFL thickness was compared between two groups of cases and controls. In addition, the correlation between VEP P100 latency and RNFL thickness in four quadrants of superior, nasal, inferior and temporal fields was estimated by spearman correlation coefficient. RNFL thickness between the patients with history of clinically isolated syndrome (CIS) was also compared to other two subgroups of RRMS and SPMS.Results: There was significant negative correlation between VEP P100 latency and RNFL. In all four quadrants, with increasing VEP latency, RNFL thickness decreased. Furthermore, there was significant correlation between P100 latencies and mean RNFL thickness [Pearson correlation coefficient = -0.527, P < 0.001; RNFL (mean) = (-0.44 ± 0.087) × P100 + (153.6 ± 10.94)]. Comparing RNFL thickness between three groups of CIS, RRMS, and SPSM, no significant difference was detected in RNFL thickness (P > 0.05). Power analysis demonstrated that RNFL average had the highest area under curve.Conclusion:OCT does have good correlations with P100 latency, indicating retinal non-myelinated axonal involvement in early stages in addition to the myelinated axonal involvement. However, it cannot be used as the sole test in evaluating visual pathway in optic neuritis and complementary tests as VEPs are recommended.