In this paper we studied a extraction and topreconcentration process of arsenic in seawater by liquid -liquid phase extraction technique (APDC/MIBK/HCl) and combined with determination by atomicabsorption spectrometry with hydride generation. Parametersinfluence upon the extraction procedurewere also investigated. As5+ was reduced to As3+ by potassium iodide and ascorbic, adjusted pH 4 ÷ 5by NaOH solution then complex with ammonium pyrrolidinedithiocarbamate (APDC) 0.01% andextract with 20 ml MIBK within 10 ÷ 30 min and 20ml HCl 8%. Liquid - liquid extraction techniquetopreconcentrate the arsenic up to 5 times. Quantitation limits of method was 5.07 µg/l. The methodhas been applied to the determination of arsenic in seawater collected at South-West seawater ofVietnam. Concentration of arsenic in seawater about µg/l and Concentration of arsenic in seawaterabout some µg/l and similar with results of other authors.Keyword: Arsenic, HG-AAS, chiết lỏng – lỏng, APDC, MIBK, nước nhiễm mặn.