Antibacterial and antiplatelet activities of Acorus gramineus rhizome-derived asaronaldehyde and asaron were analyzed using platelet aggregometer and six human intestinal bacteria. Active constituent of A. gramineus rhizome was isolated and characterized as asaronaldehyde by spectral analyses. At 2 and 1 mg/disk, asaronaldehyde exhibited strong inhibition of Clostridium perfringens and C. difficile without adverse effects on growth of beneficial bacteria such as Bifidobacterium bifidum, Lactobacillus acidophilus, and L. casei. Asaron also revealed moderate growth inhibition against C. perfringens and C. difficile at 2 mg/disk, no growth-inhibiting activity was observed on B. bifidum, L. acidophilus, L. casei, and E. coli. At 50% inhibitory concentration ($IC_{50}$) value, asaronaldehyde was effective in inhibiting platelet aggregation induced by collagen ($IC_{50}$, $27.6;{mu}M$) and arachidonic acid ($IC_{50}$, $53.7;{mu}M$). These results suggest asaronaldehyde may be useful as lead compound for inhibiting platelet aggregation induced by collagen and arachidonic acid.