The electron field emission properties of carbon nanotubes (CNTs) formed by catalyst-assisted solid-state growth process from nanodiamond films were investigated. The SEM and Raman spectroscopy examinations show that the nanodiamond films were converted into carbon nanotubes when they were coated with a thin layer of Fe, Co or Ni films (∼1.0 nm) and then postannealed at high enough temperature (∼1000 °C). Precoating a catalytic metal layer is of prime importance to induce the phase transformation process. The field emission properties of nanodiamond films have been dramatically improved. The turn on field was reduced from EDia=36–44 V/μm for nanodiamond films to ECNTs=11 to 16 V/μm, whereas the electron field emission capacity was increased from Je(Dia)=110 μA/cm2 for nanodiamond films to Je(CNTS)=30 mA/cm2 after the conversion process. The electron field emission of the solid-state converted CNTs thus obtained are superior to those of the CNTs grown from CVD process due to the suppression of carbon soots formation.