Habitat selection in Black-tailed Gulls (Larus crassirostris) on Hongdo Island, Korea, was studied during the breeding period in 2002-2003. To compare topographical advantages and disadvantages on breeding, we examined two habitats on the island: rocky-cliffs (lower and edge) and grassy (upper and inside) for differences in breeding biology and feeding frequency. In rocky-cliff habitat, Black-tailed Gulls had higher clutch size, faster laying and hatching date, and higher hatching and fledging success. Topographically, rocky-cliff habitat had two advantages-it was closer to sea and difficult to access. Proximity to the sea allowed higher feeding frequency whereas difficult access restricted predators. Conversely, grassy habitat was farther from the sea and allowed easier access to predators, but a lot of grass covered nests to protect eggs and chicks. These disadvantages caused low feeding frequency and higher hatching failure. Egging by fisherman was also a disadvantage of grassy habitat because of easy access from landing places. Therefore, our results suggested that rocky-cliff habitat was more profitable habitat than grassy habitat and this profitability was related to topographical difference between habitats.