Numerous clay veins formed along fractures such as fault, joints, cracks and small fissures are found in granitic rocks in Korea. Granitic rocks of three geological stages (Jurassic, Cretaceous and Paleogene) occur as the Daebo, Bulguksa and Hoam granites, respectively. Specimens from clay veins composed mainly of illite and their host granitoids were dated using the K-Ar method. The respective ages were: Jurassic: granites 143.7Ma and 160Ma, clay veins 104Ma and 107Ma; Cretaceous: granite 133.2Ma, clay veins 93.6Ma, 84.2Ma and 84.3Ma; Paleogene: granite 39.7Ma and 35.4Ma, clay veins 27.1Ma and 23.9Ma. The ages of the clay veins in Korea are clearly much younger than those of their host granitoids.