A simple, reliable, reproducible, and efficient method to decorate multi-wall carbon nanotubes (MWCNTs) with iron nanoparticles is presented. Purified MWCNTs are first functionalized with mono-and tricarboxylic aryl diazonium salts generated in situ, then iron nanoparticles are formed using iron (II) acetate. Different characterization techniques (XPS, TEM, PXRD, and FESEM) are used to assess the properties of the resulting materials. Homogeneous distribution of iron nanoparticles on MWCNTs is evidenced with a Gaussian mean diameter of similar to 2.7 +/- 0.2 and similar to 3.8 +/- 0.3 nm for monocarboxylic and tricarboxylic functionalizations, respectively. Obtaining such a small size homogeneously distributed iron nanoparticles on MWCNTs is the main achievement of this work. Furthermore, nanoparticles based on tricarboxylic aryl diazonium functions, used for the first time to functionalize CNTs, are more crystalline and essentially in the metallic state. This opens interesting perspectives for nanotechnology. The present methodology is also applicable to large-scale preparation.