Carbon nanotubes are now becoming important nanomaterials for use in day to daylife because of their unique one-dimensional nanostructure and unusual physicalproperties1.Using carbon nanotubes in various applications raises concern regarding the safety of its use.A number of techniques have been devised for monitoring cytotoxicity towards targetcells2,3.In this study,we compare the cellular toxic response with respect to the different sizes of Single-walled carbon nanotubes(SWNTs) and investigate therelationship between size and toxicity.Different sizes of SWNTs(c-SWNTs) wereprepared by the method of mixed acid oxidation treatment and the ability of SWNTs dispersity wasincreased greatly.SWNTs with different sizes in various concentrations were contact with thetarget cells respectively.The cellular proliferation of the cells was examined using mitochondrialfunction (MTT) assay.After exposure of 24h the results showed larger particles have highertoxicity than smaller particles,while the cytotoxicity of smaller particals was stronger than thelarger particals after exposure of 48h.And the results revealed that c-SWNTs are harmful to cells ina dose- and time-dependent responses manner.