Liangdang County is a county under the administration of Longnan City, in the southeast of Gansu Province of China. It has a land area of 1,374 km2 (531 sq mi), and a population of 50,000 in 2004. The postal code is 742400. The county seat is located in Chengguan Town. In 2017 it was the poorest county in Gansu.