Abstract: A costeffective approach is supposed to insitu monitor the cure of epoxy resin with common low refractive index fiberoptic sensor. A fiberoptic refractive index sensor was made by removing a short region of the cladding of a low refractive step index optical fiber. The sensor was used to insitu monitor the cure of epoxy resin by measuring the refractive index change of the resin during the cure process. The model of the sensor was developed on the basis of wave guide theory. A probe configuration was made by depositing a high reflective film on the endface of the fiber and mostly facilitate the tooling. Experiments have been performed to test the correctness of the model and the feasibility of the supposed approach.