This study aimed to determine mangrove forest structure and composition in Golo Sepang Village, Manggarai Barat District. Important value index and diversity index value obtained from mangrove vegetation analysis were used as indicators for determining mangrove forest structure and diversity. Transect method with square frame along the line was applied in vegetation analysis. Totally 10 lines and 30 plots were applied for getting types of mangrove, growth parametersvalue (height and diameter) and others related information. This study found that the stucture of mangrove consist of 5 familyes with 10 species, namely: Rhizophoraceae (Ceriops tagal (Perr), Rhizophora apiculata (Bi), R. mucronata Lmk., Bruguiera parviflora (Roxb.), B. sexangula (Lour) dan B. gymnorrizha (L.) Lamk.), Fabaceae (Derris trifoliata Lour), Meliaceae (Xylocarpus granatum Koen), Pteridaceae (Acrosthicum aereum Linn) and Lythraceae (Phempis acidula Forst). R. apiculata (Bi) is the most dominant species founded in 7 of 10 total sites. Two sites, Sotri and Muara Kiri, have the highest important value index (300%) for R. Apiculata species. The highest individual density is found at Sotri site, with value 1.300 tree/hectare, while the lowest density, 100 trees/hectare is found at Muara Kanan site. In diversity of mangrove, all sites were categorized as low with highest diversity index value 1,06.