Abstract: A straightforward method is presented to produce catalyst particlefilled carbon nanotubes. Based on capillarity and surface tension, a simple strategy was developed to selectively fill carbon nanotubes with Fe, Co, and Ni species using a twostep method consisting of impregnation and selective washing. By chosing the impregnation and washing solution properly, metal particles are produced exclusively inside the nanotubes. STEM and SEM investigations confirm the location of these metal particles is only inside the carbon nanotubes.