The inhibition of platelet and antiplatelet aggregation activities of sea grapes employing Ca2+-induced human platelet aggregation by Giemsa dye in a microplate assay was investigated. Flame atomic absorption spectroscopy (FAAS) measured the magnesium content of air-dried (Mg=0.4%) and fresh (Mg=0.06%) samples. Acute oral toxicity test (OECD–423) categorized sea grapes as non-toxic. Approximate inhibitory concentrations for air-dried (3.98–15.85 mg/ mL) and fresh (1–3.98 mg/mL) were pursued. Inhibitory concentration 50 of air-dried (IC50=3.64 mg/mL), in actual, (IC50=6.31 mg/mL, m=100%) and fresh (IC50=1.3 mg/mL, m=65.45%) reveal a significant difference (p<0.01). Meanwhile, the mean percentage platelet aggregation inhibition of air-dried has no significant variation with, Aspirin 6.31 mg/mL (m=100%) and 1.3 mg/mL (m=100%) (p>0.01). These values reflect that sea grapes, with high magnesium content, is a better inhibitor of platelet aggregation. Moreover, air-dried sea grapes (m=100%) produce similar inhibition of platelet aggregation with Aspirin 6.31 mg/mL (m=100%) and Aspirin 1.3 mg/mL (m=100%). Result of the mean percentage antiplatelet aggregation activity for air-dried (m=100%), fresh (m=75.55%), Aspirin 6.31 mg/mL (m=100%) and Aspirin 1.3 mg/mL (m=100%) reflect no significant variation (p>0.01). These values reveal that both air-dried and fresh sea grapes have comparable antiplatelet aggregation activity with Aspirin.