The flexural property of the Ulleung 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 ba sin formation, and therefore it has no mechanical strength during the rift ing 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 2-