Co To Island which has the ancient name Chang Son (Chang Mountain), has long been the residence of fishermen’s boats in the Northeast but has not yet become a settlement because it was always harassed by Chinese pirates.

Co To has 71 large and small islands, of which the three largest islands are Co To Lon, Thanh Lan, and Tran. Co To Island and Thanh Lan Island are located close to each other and together with smaller neighboring islands form the Co To archipelago.

Co To has a mountainous terrain. The peak adjacent to Cap Chau on Thanh Lan Island is 210 m high, and the top of the big Co To Island meteorological station is 160 m high. The part between the islands is high, surrounded by low mountains and narrow fields, along the island are small sandy beaches and coves. The soil is mainly ferret on sandstone.