Aim: Microglial activation has been implicated in many neurological diseases. In this study, we examined the effects of tetrandrine (TET), a major pharmacologically-active compound of Chinese herb Stephania tetrandra S Moore on microglial activation. Methods: The microglia pretreated with or without TET were activated by lipopolysaccharide (LPS) in vitro. Nitric oxide (NO) release, superoxide anion (O) generation, as well as TNF-α and interleukin-6 (IL-6) production by microglia were measured afterwards. Electrophoretic mobility shift assay was performed to determine whether NF-κB activity in microglia was affected by TET treatment. Results: We found that TET inhibited the LPS-induced activation of microglia by decreasing the production of NO and O, consequently affecting the release of TNF-α and IL-6 in LPS-induced microglial activation. Such suppressive effect was accompanied by inhibiting transcription factor NF-κB activation. Conclusion: Our results suggest that TET might modulate LPS-induced microglial activation by inhibiting the NF-κB-mediated release of inflammatory factors. © 2008 CPS and SIMM.