Shipborne gravity data are analyzed to investigate crustal structure under Dok Island and its surrounding seamounts located in border of Ulleung Basin and Oki Bank in the East Sea. Relatively low free-air gravity anomaly compared with the volume of seamounts may be explainable by isostatic compensation. From 1 st to 3rd Dokdo Seamounts, the decrease of free-air and Bouguer gravity anomalies implies the different degree of isostatic compensation, crustal thickness or/and density contrast. 3-D gravity modelling shows that seamounts have the mirror roots for regional Airy isostatic compensation, and from Ulleung Basin to Oki Bank, Moho discontinuity deepens and the density of crust is decreases. The results infer that study area is transitional zone from thin oceanic to thick continental crust. The depth of Moho discontinuity is about 15∼16 km, which may be interpreted as an uplifting of Mantle to shallow depth comparing with other borders of the Ulleung Basin.