The flexural property of the Uleng back-arc basin was examined along three deep seismic profiles that transect the basin margin. Our method assumes that the crust is stretched according to Airy¡¦s isostasy during basin formation, and therefore it has no mechanical strength during the rifting stage. However, upon completion of rifting and cooling, the extended crust gains a finite strength and response to sediment loading in elastic manner. For a given elastic thickness of the crust, we construct a simple 2D model and search for the elastic thickness that best matches the observed gravity anomalies. Of the three profiles, two gave a reasonable fit with the best matching elastic thickness of about 8-10 km. This value is substantially lower than major passive margins, but comparable to young marginal basins that have undergone rapid extension and is largely consistent with the model of Ulleung Basin development. The cause of large gravity anomaly misfit along the third profile across the southwest corner of the Ulleung Basin is unclear. However, our preferred explanation is that a mechanism other than extension and subsidence, such as thrusting and strike-slip faulting related with the late-stage closure of the Ulleung Basin, may be responsible for the crustal structure there.