Two kinds of carbon coating on carbon nanotubes (CNTs) after decoration with nanoparticles were observed by scanning transmission X-ray microscopy with a concurrent characterization of the electronic structure, revealing the complex compositions in CNTs after chemical modification. Moreover, a sandwich structure shown as CNT-nanoparticle-coating can be created with the exposure to X-ray for CNTs decorated with nanoparticle outside the wall. The coating shows an effective way for site-selective modification of CNTs with various carbon structures. (C) 2013 Elsevier B.V. All rights reserved.