In this study, we describe the effect of diuron, a photosynthesis-inhibiting herbicide, on growth, photosynthetic pigment contents, carbohydrate, protein, amino acids, proline, nitrate reductase and some antioxidant enzymes in cell suspensions of Chlorella vulgaris for seven days. The growth (cell number & dry weight) was seriously affected even with lowest diuron dose (0.1 µM). Photosynthetic pigment contents (chl-a, chl-b & caroteniod) of diuron-treated algae, was more sharply decreased as compared to control. Diuron induced oxidative stress as indicated by the alteration of ascorbate oxidase. Nitrate reductase (NR) activity in the volume of algal suspension was significantly decreased with increasing diuron doses. Guaiacol peroxidase was un-detectable in diuron-treated and un-treated, which may be due to hypersensitivity of C. vulgaris towards toxic compounds. Proline, total amino acids, carbohydrate and protein contents of algal suspension decreased significantly in response to diuron treatments. All results showed a negative correlation with treatments except ascorbate oxidase activity.