Acorus calamus is an indigenous hydrophyte used as phytoremediation and water purification as well as medicinal herb. Irradiation of gamma ray was performed to investigate the effect on radioresistance of A. calamus on in vitro. Various doses (20-500 Gy) of gamma rays were irradiated to in vitro plantlets. The effect of irradiation was studied on survival rate, shoot formation and growth rate. The survival and multiplication rate was remarkably decreased with the increase of radiation dose. The growth of plantlets was significantly increased in comparison to the control at low doses. The hormetic effect of radiation was appeared in A. calamus. It was significantly decreased or interrupted above 250 Gy. The mean 50% lethal dose (LD50) was extremely high as 240 Gy. We conclude that A. calamus had remarkably high radioresistance compared with other plant species.