Aims: The purpose of this study was to determine the inhibition and anaerobic biodegradation of benz[a]anthracene-7, 12-dione (BaAQ) via the specific methanogenic activity (SMA) test. Materials and Methods: In this study, 120 mL vials were filled with given concentration of BaAQ, anaerobic biomass and substrate. Each batch experiment was lasted 13 to 26 days. The inhibition effects of BaAQ with concentrations of 0.5, 5, 25, 50, 100, 250 and 500 mg/L on the methanogenic process was investigated in the presence of volatile fatty acids including acetic, butyric and propionic acids. Results: In some eatch test, the gas production was stopped after 13 days (312 h). Therefore, 312 h was considered as compared base of gas production. After this time, methane production cumulative rate was calculated for the each SMA. After 13 days (312 h) the lowest and the highest cumulative methane has been produced (per mL) at the presence of concentrations of 250 and 0.5 mg/L of BaAQ, respectively. Conclusion: The results of this study showed that the BaAQ with concentration of 250 mg/L has more inhibition effect on methane production rather than other concentrations (even of 500 mg/L). Therefore, we should not always expect that higher concentrations of toxic compounds had more inhibitory effects than low concentrations of them.