In this study, we investigate the antimicrobial activity of treated Multi Walled Carbon Nanotubes (MWNTs) by oil toward gram negative bacteria Escherichia coli(E.coli) and treated MWNTs is performed according to a chemical procedure introducing -COOH groups on their surface . Structural and morphological characterizations of samples have been carried out using SEM and FT-IR spectroscopy. In addition, the SEM images prove the antimicrobial activities of treated MWNTs to bacterial cells by adhesion activity of E.coli on MWNTs surface. Since the functionalization of Multi Walled Carbon Nanotubes (MWCNT-COOH) can be used as effective antimicrobial materials and indicate that severe cell membrane damage by direct contact with MWNTs is the likely mechanism responsible for the toxicity to the model bacteria.