In the present study the nanocapsulation of Haloxyfop-R-methyl in poly(methyl methacrylate) was successfully performed. Poly(methyl methacrylate)/Haloxyfop-R-methyl nanocapsules were synthesized using a miniemulsion method and their surface morphology was studied by scanning electron microscopy and transition electron microscopy. The chemical characterization of nanocapsules was done by FT-IR spectroscopy. The herbicide loading and encapsulation efficiency were also analyzed for the herbicide-loaded nanocapsules. In order to evaluate the toxic effects of nanocapsulated herbicide and pure herbicide on Lemna minor L., some physiological effects of these two compounds were investigated. The plant growth rate, photosynthetic pigment contents, and activities of antioxidant enzymes were evaluated as indices to assess the effects and toxicity of the encapsulated Haloxyfop-R-methyl and its nonencapsulated form on Lemna minor. The obtained results confirmed that the negative effects of encapsulated Haloxyfop-R-methyl on the physiological parameters of Lemna minor were less than the effects of the pure herbicide. In the case of antioxidant enzymes activities, it was shown that all concentrations of the two examined groups led to the remarkable induction of superoxide dismutase activity as compared with the control sample. Possibly, the enzyme played an important role in the plant's resistance to the existence of the studied contaminants.