Koh Thmei and neighboring island Koh Seh comprise the eastern third of the Ream National Park, situated in the South-East corner of Prey Nob District, Sihanoukville province. At the narrowest point, due west, the island is a mere 300 meters off the mainland. Yet all transports arrives via the Prek Toek Sap estuary from Koh K'chhang fishing village to the north, a distance of around 3 km. Roughly pentagonal shaped with four sides of similar length, a shorter and rugged fifth side in the South, the inner coastline is dominated by extensive mangrove marshes. Along the sea-facing coast, sizable beaches can be found, littered with shells, the sands of rather coarse composition. Still, due to the unspoiled, pristine state, dense, lush and primeval vegetation, its general remoteness and very sparse population, Koh Thmei is considered one of the most ecologically esteemed Cambodian islands among its visitors. The island's center is roughly the highest point with two main peaks, separated by a river {Prek Koh Krabei}. The peaks rise to over 100 meters and drain in all directions, lending Koh Thmei the shape of a gentle mountain. The island's moderate elevation enables it to retain enough water for a few little rivers, creeks and estuaries.