We investigated the antithrombotic effect of Acorus gramineus Soland (A. gramineus) from Korea and Acorus tatarinowii Schott (A. tatarinowii) from China in a rat model of arterial thrombosis induced by ferric chloride ($FeCl_3$). Thirty minutes prior to a 35% $FeCl_3$ application, Sprague Dawley rats were intraperitoneally injected with saline, A. gramineus and A. tatarinowii (100 mg/kg), respectively. Occlusion time of rats injected with A. gramineus was delayed significantly compared to that of the vehicle and A. tatarinowii. Thrombus weight was meaningfully decreased in rats injected with A. gramineus compared to the vehicle. Additionally, A. gramineus inhibited collagen fiber damage in vessel compared to the vehicle, but A. tatarinowii did not show a significant effect. Our results show that A. gramineus and A. tatarinowii from the same genus have different antithrombotic effects, and especially A. gramineus has a better antithrombotic effect than A. tatarinowii.