Deca-brominated diphenyl ether (BDE-209) is a new kind of persistent organic pollutant, which may have multi-physiological toxic effects on the nervous system, reproduction, development, etc. of an organism. In the present study to assess toxicity of BDE-209 at the molecular, biochemical and physiological levelsCaenorhabditis elegans (C. elegans) was chosen as a model organism. The physiological experiment performed in this study was to determine effects of the substance on spawning rate of parental generations and filial generations of C. elegans, and life span and cell apoptosis of the parental generations; the biochemical experiment was to monito r cell apoptosis of C. elegans with the acridine staining method; and the molecular-level experiment was to determine changes in antioxidation genes, such as superoxide dismutase-3 gene (sod-3) and quasi p53-1 protein gene (cep-1), and xenobiotic metabolism enzyme, such as cytochrome P450 35a2 gene (cyp35a2) and glutathione-s-transferase gene (gst-1) with the Real-Time PCR method, so as to evaluate toxicity of BDE-209 at the physiological and molecular levels. Results show that compared with control group, exposure to a low dosage (5 mg kg^-1) of BDE-209 did not have much effect on spawning rate of the parental generation, but did on that of the filial generations, particularly on that of the third filial generation, which dropped by 61.5%. Exposure to medium (20 mg kg^-1 ) and high (30 mg kg^-1) dosage of BDE-209 did inhibit their spawning rate (p < 0.05) significantly, reducing that of the third filial generation by 66.7% and 89.3%, respectively. It is clearly shown that both long-time exposure to a low dosage (5 mg kg^-1) and short-time exposure to a high dosage (30 mg kg^-1) of BDE-209 could affect brood size of C. elegans. The experiments on life span and cell apoptosis demonstrate that exposure to a low dosage (5 mg kg^-1) did not have much effect, but exposure to medium (20 mg kg^-1 ) and high (30 mg kg^-1) dosages of BDE-209 did reduce mean life span of C. elegans; and exposure to a high dosage cause apoptosis around the gential area. Real-time PCR reveals that exposure to a low dosage ( 5 mg kg^-1) of BDE-209 significantly induced the expression of stress-related genes such as sod-3, cep-1 and xenobiotic metabolism enzyme cyp35a2 genes, but inhibited the expression of xenobiotic metabolism enzyme gst-1 genes, while exposure to a high dosage (30 mg kg^-1) inhibited the expression of cep-1, cyp35a2 and gst-1, but increased the expression of sod-3 to an extent that was not so high as the former. However, it is quite obvious that the expression of gst-1 was always inhibited in either case. These findings indicate that, when exposed to a low dosage of BDE-209 (5 mg kg^-1) for a short time, C. elegans would react to oxidative stress, trying to repair the damage through regulating the expression of related anti-oxidation genes. In this case, C. elegans would be much affected in life span and spawning rate, however with the exposure going on, a low dosage (5 mg kg^-1) of BDE-209 would significantly reduce the spawning rate of C. elegansand a high dosage of BDE-209（30 mg kg^-1）would cause oxidative injury and cell apoptosis to organs of of the C. elegans, and reduce its oviposition capacity.