Multi-walled carbon nanotubes (MWCNTs) were irradiated by gamma rays in air and epoxy chloropropane (ECP) with an absorbed dose 200 kGy We found that MWCNTs showed an opposite behavior in structural change when irradiated in the two different media Gamma-ray irradiation decreased the inter wall distance of MWCNTs and improved their graphitic order in air, while irradiation in ECP increased the inter wall distance of MWCNTs and disordered the structure (C) 2010 Elsevier Ltd All rights reserved