Seagrasses, marine flowering plants, are widely distributed along temperate and tropical coastlines of the world. Seagrasses have key ecological roles in coastal ecosystems and can form extensive meadows supporting high biodiversity. Till now, seven seagrass species were found in Ly Son island belong two family Hydrocharitaceae and Cymodoccaccae. Including Thalassia hemprichii, Halophila ovalis, Halophila minor, Cymodocea rotundata, Halodule pinifolia, Syringodium isoetifolium and Halodule uninervis. In there, Thalassia hemprichii and Cymodocea rotundata species are dominaced about area and covering. Total area of seagrass bed is estimated by GIS technology approximately 188,9 ha. The south-west and south-east of Ly Son island are the most popular of the seagrass. In recents years, the seagrass bed in Ly Son has been serious decline by many reasons: exploiting sands, harbour construct.