The curing process of a dicyanate resin system was investigated by a positron-annihilation lifetime technique (PA). Based on the observation of PA, it was shown that the size of free volumes of cured resins became larger as the curing reaction proceeded. This result was supported by a change in the physical properties related to the size of free volumes, such as a decrease in the specific gravity, a decrease in the dielectric constant and an increase in the water pick-up rate with increasing the decree of the cure. It was therefore found that the curing reaction of dicyanate resin proceeded through a reaction of cyanate groups to form a larger size of free volumes.