Diamond films with a thickness ranging from 5 to 10 μm have been deposited on Si(100) substrates using hot filament CVD from a CH4-H2 mixture. These diamond films have been etched via plasmas generated from H2, O2, Ar and CF4 as well as via diffusion etching using transition metals, such as Co and Mn. It was found that CVD diamond is very resistant towards Ar and CF4 plasmas, while H2, O2 plasma are more aggressive towards diamond through chemical reaction between carbon and atomic hydrogen and oxygen. Columnar structure was formed after O2 plasma etching, while H2 plasma resulted in anisotropic diamond etching. Diamond etching via diffusion reaction with transition metals was not effective.