The steam distillate obtained from Thujopsis dolabrata var. hondai sawdust was fractionated by centrifugal thin-film evaporation, and the fractions were then investigated for antiplatelet activity using washed rabbit platelets. The biologically active constituent of T. dolabrata var. hondai sawdust was isolated by silica gel column and HPLC chromatographies and characterized as carvacrol by various spectral analyses. Carvacrol inhibited platelet aggregation induced by collagen, arachidonic acid, and platelet activating factor with IC$_{50}$ values of 12.6, 2.5, and 385.3 $mu$M, respectively. However, carvacrol had no effect on thrombin, calcium ionophore A23l87, or phorbol l2-myristate l3-acetate induced platelet aggregation. Carvacrol was a much more potent inhibitor, as antiplatelet agents, compared with aspirin. These results suggest that carvacrol isolated from T. dolabrata var. hondai sawdust may be useful as a lead compound for inhibiting arachidonic acid-induced platelet aggregation.