This study proposes a real-time monitoring method for degree of cure (DOC) of curing resin by refractive index measurement using a single-mode optical fiber based on the Fresnel reflection at the fiber end. First, we particularly present an approach for measuring refractive index of resin in real time without the effects of unexpected backward reflection. We applied this approach to measurement of refractive index of an epoxy resin during cure process, and experimentally confirmed that the change of the refractive index was stably measured in real time. Next, we proposes a new approach to calculate DOC from change of refractive index by eliminating effect of temperature change on refractive index of curing resin. This approach can quantitatively evaluate the DOC in real time, which is not obtained by conventional methods, and provide information in the small region at the fiber end. The experimental results of monitoring cure process of the epoxy resin showed that the DOC could be stably measured by this approach without effect of the temperature change. Furthermore, the DOC obtained by this approach was compared with the DOC curve by the thermal analysis using differential scanning calorimeter. These experiments revealed that the DOC by the present approach represented the curing condition of the resin regardless of the cure pattern and was easily translated into the DOC by the thermal analysis. The present approach of real-time quantitative evaluation of the DOC can be applied to fast cure reaction as well as slow reaction demonstrated in this paper.