Abstract: Carboxylated multi-wall carbon nanotubes (MWCNT-COOH) were first modified by 1,3-diphenylpropane-1,3-dione to form a MWCNT-ketone and then by hydrazine to produce MWCNT-pyrazole. All modified samples were characterized by FT-IR, Raman spectroscopy, SEM, elemental analysis, TGA, DTG. Their interactions with living cells were investigated. The toxicity of the samples was evaluated with unrestricted somatic stem cells and viable cell numbers were measured by reduction of living cells with 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Results indicated that there is one diketone group in MWCNT-ketone and one 3,5-diphenyl pyrazole group in MWCNT-pyrazole per 28 and 72 carbon atoms of MWCNTs, respectively. MWCNT-ketone had the lowest toxicity in all samples investigated.