The study on species diversity of polychaeta in Phan Thiet Bay was carried out in May 2009. A total of 3, 767 individuals of 11 6 taxa, 13 orders and 38 families were found, of which Magelona pectinata Nateewathana & Hylleberg, 1991 was a newly recorded species of Polychaeta in Vietnamese waters. Spionidae was the richest family with 15 species, Capitellidae and Syllidae were predominant families with 8 and 7 species respectively. Density and biomass of polychaeta in Phan Thiet Bay differed between sites and transects, highest in the north adjacent to the coast and decrease by offshore and south of Bay.