Carbon nanotubes (CNTs) were grown on Fe needle by a modified microwave plasma enhanced chemical vapor deposition (MPECVD) process, in which the plasma was induced in the vicinity of the tip of Fe needle through the interaction of microwave and Fe needles. The spectra of plasma were in situ examined, indicating that Ar / CH4 ratio and total pressure remarkably affect the characteristics of the plasma spectra, which, in turn, correlated intimatedly with the electron field emission properties of CNTs. The presence of singly ionized carbons (CII) in the plasma is presumed to be the main factor facilitating the growth of CNTs, possessing good electron field emission properties. The maximum electron field emission current density is 8 mA/cm2 for carbon nanotubes grown at 10% CH4 and 300 Torr total pressure. The corresponding turn-on field (Eo) is 20 V/μm. Restated, the electron field emission properties of CNTs can be optimized by controlling the spectral characteristics of plasma.