The effect of argon (Ar) ion treatment on the field-emission (FE) properties of amorphous carbon nanotubes (aCNTs) and crystalline multiwall carbon nanotubes (MWCNTs) is reported in this article. Under optimum Ar-ion irradiation conditions, the FE properties of carbon nanotubes (CNTs) have been found to improve. The turn-on field of MWCNTs downshifted from 2.81 to 0.85 V μm−1 and from 9.17 to 3.27 V μm−1 for aCNTs. Improvement of the FE behavior is associated with the morphological modifications of CNTs, as a result of Ar-ion irradiation. Ar-ion bombardment creates defect sites on the CNT surface as confirmed by various techniques such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). We have proposed that the defects caused modifications of the CNT surfaces resulting in an effective increment of the field-enhancement factor β.