In this paper, after reviewing the principles and theory of operation for a Fiber Bragg grating (FBG)-based relative humidity sensor, we have investigated the impact of parameter change on the characteristics of the sensors. Parameters such as the number of gratings, the difference in refractive indices of the layers, and the refractive index distribution or refractive index modulation were examined. The characteristics of interest were output linearity and sensitivity, and the theoretical and simulation results were presented. The results show that the best linearity is obtained when the number of layers is at its minimum while higher sensitivity demands more layers and a higher contrast in refractive indices of the layers. Increasing sensitivity reduces the spectrum over which the measurement is feasible.  Refractive index modulation reduces the side lobes in the reflectivity spectrum, but does not have considerable impact on linearity or sensitivity.