An experiment was conducted to evaluate chemical control of duckweed (Lemna minor) and its effect on water quality and herbicide residue. Paraquat, glyphosate and metsulfuron-methyl (MSM) were applied in different doses on Lemna minor mat in water tank. Paraquat 0.5 kg/ha resulted in 100% control of Lemna minor in 15 days while metsulfuron-methyl (MSM) and glyphosate resulted 100% control in 30 days. Water quality in relation to dissolve oxygen and pH were affected by all treatments as compared to the untreated control. Low pH was found in all treated water tanks compared to untreated control. Residues of paraquat in water were 0.11 to 0.39 ppm at 0.25 to 1.0 kg/ha application rate at 0 day, which was reduced to 0.001 to 0.006 ppm at 60 days after application (DAA). Residues of MSM in higher dose (0.016 kg/ha) ranged from 0.56 at 0 day to 0.070 ppm at 60 DAA, but it could not be detected in lower dose at 0.008 kg/ha at 60 DAA. Glyphosate residue in higher dose (1.5 kg/ha) ranged from 0.14 ppm at 0 day to 0.017 ppm at 60 DAA.