This paper studied the accumulation and scavenging of active oxygen radicals, injury of photosynthetic system and membrane system, and changes of antioxidant enzyme system in Thuidium cymbifolium cells under single and combined stress of Pb and Ni. Under low concentration of Pb and Ni (Pb < 0.1 mmol x L(-1), Ni < 0.01 mmol x L(-1)), the chlorophyll content of T. cymbifolium increased; while under high concentration of Pb and Ni (Pb > 0.1 mmol x L(-1), Ni > 0.01 mmol x L(-1)), it was in adverse. There was a dose-dependent accumulation of reactive oxide species (ROS) and malondialdehyde (MDA) under combined Pb and Ni stress. The superoxide dismutase (SOD) and catalase (CAT) activities decreased while the peroxidase (POD) activity increased with increasing Pb and Ni concentration, indicating the important role of POD in eliminating ROS under Pb and Ni stress. The dose-dependent change of MDA content and CAT activity under Pb and Ni stress suggested that T. cymbifolium could be used as a biomarker in pollution monitoring of these two heavy metals.