Mangrove mempunyai fungsi yang sangat penting baik fungsi ekologis maupun fungsi ekonomis, sehingga pengelolaan dan pemanfaatan harus diperhatikan agar terjaga kelestariannya. Dalam upaya menjaga kelestarian hutan mangrove perlunya informasi struktur keanekaragaman jenis vegetasi mangrove melalaui analisis vegetasi mangrove. Penelitian ini bertujuan untuk mengetahui struktur keanekaragaman jenis, komposisi dominansi (INP), nilai indeks keragaman dan kerapatan jenis vegetasi mangrove. Penelitian ini dilakukan di Kecamatan Sinaboi pada 4 (empat) Desa, metode penelitian yang digunakan adalah metode jalur (transek) dengan penetapan petak ukur secara systematic sampling. Pengukuran langsung dilakukan dilapangan mengunakan metode jalur tegak lurus dari tepi sungai ke arah darat, peletakan petak ukur pada jalur dengan jarak antar jalur 500 meter dan jarak antar petak 300 meter. Petak ukur berbentuk persegi dengan ukuran 2 x 2 meter tingkat semai, 5 x 5 meter tingkat tiang, 20 x 20 meter tingkat pohon. Analisis data mengunakan perhitungan analisis vegetasi (Mueller - Dombois dan Ellenberg) dan nilai indeks keragaman jenis (simpson) berdasarkan data yang diperoleh kemudian dilakukan perhitungan nilai pentingnya untuk mengetahui komposisi dominansi vegetasi mangrove. Analisis hasil bahwa struktur keanekaragaman jenis vegetasi mangrove pada 4 (empat ) Desa tidak menunjukkan perbedaan yang signifikan rata-rata dijumpai 10 sampai dengan 11 jenis vegetasi mangrove. Hasil perhitungan nilai indeks keragaman Simpson (D) Desa Kuala 0.81, Desa Bagan Tanjung 0.84, Desa Sungai Bakau 0.87dan Desa Ujung Simbur 0.83. Kerapatan jenis tertinggi pada tingkat pohon, tiang dan semai dari family Avicenniaceae. Komposisi jenis dominansi vegetasi mangrove di Desa Kuala pada tingkat pohon, tiang didominasi jenis Rhizophora parviflora dengan INP 136.62 dan 71.09, tingkat semai didominasi oleh Rhizophora mucronata dengan INP 54.61. Desa Bagan Tanjung tingkat pohon didominasi jenis Avicennia marina dengan INP 91.65, tingkat tiang, semai didominasi Rhizophora mucronata INP 64.46 dan 46.86. Desa Sungai Bakau dan Desa Ujung Simbur tingkat pohon, tiang dan semai didominansi dari family Avicenniaceae dengan masing-masing INP 76.07, 55.93, 38.25 dan 89.90, 92.77, 51.64.

Mangrove holds highly important function on both ecological and economical functions, thus its management and utilization needs to be closely supervised to maintain its conservation. In preserving mangrove conservation structural information on mangrove vegetation diversity by mangrove vegetation analysis will be required. This research is proposed to identify structural diversity on genus, dominated composition (INP), diversity index, and density of mangrove vegetation. This research is performed at Sinaboi Sub-District within 4 villages. Raw (transect) method with plot-size establishment of systematical-sampling is employ as research method. Field-based direct measurement employs vertical line method from river edges to ground, plot size placed in lines with range 500 m and plot interval as 300 m. Plot size level classified into square with 2x2 m for seedling, 5x5m for stand, 20x20 m for tree. Collected data analyzed by both vegetation analysis (Mueller – Dombois and Ellenberg) and diversity index value (Simpson) followed by calculated the important value to acknowledge the dominated composition of mangrove vegetation. The analysis result reveals that mangrove vegetation diversity at 4 villages does have significant differences with only 10 to 11 genus that founded. The calculation result from Simpson (D) diversity index presents Kuala Village as 0.81, Bagan Tanjung Village as 0.84, Sungai Bakau Village as 0.87 ang Ujung Simbur Village as 0.83. Avicenniaceae family has the highest density for tree, stand and seedling levels. Dominated-composition of mangrove vegetation at Kuala Village for tree, stand and seedling levels is dominated by Rhizophora parviflora with INP 136.62 and 71.09, respectively, while seedling level is dominated by Rhizophora mucronata with INP 54.61. at Bagan Tanjung, tree level is dominated by Avicennia marina with INP 91.65, while for stand and seedling level is by Rhizophora mucronata with INP 64.46 and 46.86, respectively. At Sungai Bakau Village and Ujung Simbur Village, Avicenniaceace family is dominated at tree, stand and seedling levels with INP 76.07, 55.93, 38.25 and 89.90, 92.77, 51.64, respectively

Kata Kunci : Mangrove,Keanekaragaman,Komposisi, mangrove, diversity, composition