An inductively coupled plasma mass spectrometry (ICP-MS) instrument equipped with a dynamic reaction cell (DRC) was used for the determination of Arsenic in seawater sample. The potentially interfering 40Ar35Cl+ ion at the arsenic mass m/z=75 were reduced in intensity by approximately 3-4 orders of magnitude by using 0.3 mL/min NH3 as reaction cell gas in the DRC while a q value of 0.6 was used. The limits of detection for 75As was 0.02 μg/L. The cation exchange resin was used for the removal of Ca and Na matrix. This method was applied to the determine As in NRCC CASS-4 seawater reference sample. The result for the reference sample agreed satisfactorily with the reference value. The external calibration method were used for the determination of arsenic in sample. The combined uncertainty calculation is performed according to the ISO/GUM guidelines. The determined mean concentration and expanded uncertainty of calcium was 1.11±0.16 μg/L.