The structure of free living marine nematode assemblages in the West Sea, Vietnam was firstly carried out in October 2007. The objectives of this research are originally to find out . the composition, densities and biodiversity of the nematode communities in The West Sea. The primitive results of research from 7 sampling stations showed that 93 genera of 22 families, 7 orders of phylum Nematoda were recorded to present in this area. In nematode communities of West Sea, order Monhysterida present as highest dominant in the number of genera (27 genera or 29 percent of total). Chromadorida are also high with 15 genera (or 16.1 percent of total), Triplonchida is lowest number of genus in the communities. Genera Daptonema develop dominantly in two stations T2 and T5. In the other stations, this position was replaced by Thalassomonhysterh in T1, Terschellingia in T3, Vasostoma in T4, Gomphionema in T6 and Neochromadora in T7. Genera richness ranges from 11 - 44 genera and densities range 40 - 180 inds/100 gram sediment. In each sampling stations, number. of genera and densities are not equal, they went high in some stations as T2, T3 and T6, T7 but fell lowest in station T4. The nematode assemblages seems increase densities from the East to the West. The biodiversity indices as Margalef (d), Shanon - Weiner (H') were calculated to show a scenario with high diversity in which d index ranging from 2.84 4.81 and H' ranging from 2.71 - 8.26.