The residues of various heavy metals and pesticideshave been frequently detected in water environments, however, theirjoint toxicity is different from individual single toxicity. We evaluated the effects of single and co-exposure of organophosphorus pesticide chlorpyrifos and heavy metal Cu on acute toxicity, cellular permeability and anti-oxidative stress of freshwater green algae. The 72h-EC₅₀ for single exposure of Cu and chlorpyrifos to chlorella were 0.68 and 12.71μmol/L, respectively. The chlorophyll content of algal cells was decreased accordingly with the increasing concentration of Cu and chlorpyrifos, however, the cell permeability was increased with the higher concentrationof Cu and chlorpyrifos. The reactive oxygen species (ROS) and antioxidant enzymes were significantly induced in algae cells. As revealed by the additive index method, the joint effec of Cu and chlorpyrifos on acute toxicity wasin antagonistic mode, in consistent with the ROS production and antioxidant enzyme in chlorella vulgaris.