Moisture absorption of GFRP degrades mechanical properties such as compressive and fatigue strength. Monitoring of moisture absorption of GFRP structures is useful to assure the structural integrity. Conventional moisture measuring methods for various materials is inappropriate for GFRP because of its inhomogeneity and low moisture absorption ratio. In this study, a new method is proposed to measure the moisture absorption ratio of GFRP laminates by using a polymer film capacitance sensor which is embedded between GFRP plies. The dielectric constant of the film (polyimide) depends on the moisture absorption ration of surrounding resin. The moisture absorption ration of GFRP is estimated indirectly from measuring the dielectric constant of the film. At first, the relationship between the moisture absorption ratio and the dielectric constant change of an ideal capacitive film sensor itself was investigated. Then, an epoxy specimen with a flexible print film sensor in the midplane was immersed in water, and the moisture absorption ratio of the specimen and the capacitance change of the sensor were monitored. As a result, it is shown that the moisture absorption ratio of GFRP laminates can be easily measured by the proposed method.