In this study, the sensitivity of dielectric sensors for the on-line cure monitoring of resin matrix composite materials has been analysed by the FEM (finite-element method) and compared to the experimental results. Using the results of the analysis, the equation for the capacitance of the sensor as well as the effective range for which the electric field affects the medium are obtained. Also, a more sensitive sensor design than the usual planar interdigital capacitor is suggested using three-dimensional FE analysis and experiment on the sensor types. It is found that the fabricated sensor and the electric circuit for on-line cure monitoring can be used for the smart manufacturing of resin matrix composite materials.