Five microalgal strains, selected and successfully isolated to a unialgal state from mangroves trees in Xuan Thuy National Park, were identified as Navicula tuscula VACC-001, Chaetoceros muelleri VACC-005, Amphiprora alala VACC-007, Chlorella vulgaris VACC-012 and Nannochloropsis oculata VACC-017, based on morphological properties and 18S rDNA sequence analysis. ASW medium was found to be best suitable medium for their growth and biomass production. The fatty acid profile of isolated strains showed taxonomic characteristics. C. muelleri and N. oculata had the highest concentration of polyunsaturated fatty acids (PUFA), accounting for 36 - 50 percent of the total fatty acids with a ratio of w-3/w-6 ranging from 2.5 - 4 and a high level of eicosapentaenoic acid (EPA). A. alata had a significant proportion of arachidonic acid (AA) and docosahexaenoic (DHA) whereas C. vulgaris was found to have a high concentration of essential fatty acids and N. tuscula was at the high level of 14:0 + 16:0 acids, which was necessary for rapid development of larvae. Carbohydrate content of isolated strains was around 5 - 16 percent dry weight. The results suggest their high potential for use as mixed food in bivalve larvae aquaculture due to algal size, supplementary nutrition value and feasible biomass production.