Multi-walled carbon nanotubes (MWNTs) irradiated with γ-rays were chemically modifified with glucosamine and decylamine, respectively. The cytotoxicity of ,Tetrahymena pyriformis, exposured to irradiated MWNTs, MWNTs modifyed with glucosamine and decylamine (denoted by G-MWNTs and D-MWNTs) was carried out. The results indicated that MWNTs modified with different functional groups exhibited different bioactivity to ,Tetrahymena pyriformis,. D-MWNTs showed a significant toxicity to ,Tetrahymena pyriformis, and the viability of cells reduced to 46.2%. And G-MWNTs promoted significantly the growth of cells, with the viability being up to 308%. More interestingly, it was shown that the toxicity and bioactivity of modified MWNTs to cells was changed along with the contents of functional groups bound on the surface of MWNTs. The studies may supply a possible strategy for enhancing or reducing the toxicity of MWNTs to meet certain applications in biological and medical field.