Two known farnesylacetone derivatives (1 and 2) were isolated from the Korean brown alga Sargassum sagamianum off Jeju Island, Korea. Compounds 1 and 2 were identified as (5E,10Z)-6, 10, 14-trimethylpentadeca-5, 10-dien-2, 12-dione and (5E,9E,13E)-6, 10,4-trimethylpentadeca-5,9,13-trien-2,12-dione, respectively, by comparison with the literature data. Compounds 1 and 2 showed moderate acetylcholinesterase and butyrylcholinesterase inhibitory activities with $IC_{50}$ values of 65.0∼48.0 and 34.0∼23.0 $muM$, respectively.