This experiment was conducted to determine selective mechanism of cyhalofop-butyl ester ((((R-butyl 2-(4-(4-cyano-2-fluorophenoxy) phenoxy) propionate)) between rice and Echinochloa crus-galli. 100ppm of cyhalofop-butyl ester inhibited over 90% of seedling growth of E. crus-galli when applied at 3 leaf stage and complete inhibition was observed at 180ppm applied at the 4 leaf stage, but rice(Chucheongbyeo) was not inhibited by cyhalofop-butyl ester even at 230ppm, regardless of its growth stages(3, 4, 5 and 6 leaf stages). Cyhalofop-butyl ester applied through stem at 10 and 50ppm moved most rapidly to the meristem and resulted in the highest injury on plant height, root length and fresh weight of E. crus-galli. compared with root or leaf application. Seedlings of rice and E. crus-galli at 3 or 4 leaf stage were dipped in 180ppm of cyhalofop-butyl ester solution for 1 minute and aboveground parts of E. crus-galli and rice were removed immediataly after dipping treatment. Regrowth of E. crus-galli was inhibited by the herbicide by 41.7%, but no inhibition was observed in rice. Further, content of chlorophyll reduced to 18.7% of the untreated control, showing appearence of almost being killed, but no effect on chlorophyll content of rice was observed.