Triple-walled carbon nanotubes (TWNTs) with three concentric cylindrical graphene layers have been selectively synthesized for the first time from decomposition of ferrocene encapsulated inside double-walled carbon nanotubes, and were identified by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy; HRTEM observations reveal that the formation of inner tubes of TWNTs follows a base-growth mechanism.