Nanoscience emerged in the last 1980 s and is developed as one of the most promising new science and technology in the 21st century. With the increasing widespread application of nanomaterials,their health risk has been greatly increased and researches on its biological safety are imperatively needed. In this paper,the toxic influential factors,the cytotoxicity mechanism of nanomaterials and the evaluation methods on cytotoxicity of nanomaterials in vitro were elucidated in detail. Simultaneously,the latest developments on the toxicity of nanomaterials and the security assessment of nano technologies were also systematically discussed.