We demonstrate that the quality of the as-grown single-walled carbon nanotubes (SWCNTs) can be effectively improved by the addition of the B ingredient in the Fe/MgO catalyst. The as-grown SWCNTs were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The SWCNTs prepared by the pure Fe/MgO catalyst have relatively low graphite crystallinity and are coated by much amorphous carbon. The intensity ratio of the D- and G-bands (I-D/I-G) in Raman spectra is relatively high (0.098 for laser 532 nm and 0.075 for laser 785 nm). The SWCNTs grown from the Fe/MgO catalyst doped with 0.1 part of B have more regular graphite structure with little amorphous carbon. The I-D/I-G values reduced remarkably (0.041 for laser 532 nm and 0.040 for laser 785 nm). The effect would be attributed to the inhibitory action of the doped B on the formation of radical hydrocarbon species for the formation of SWCNTs. (C) 2011 Elsevier B. V. All rights reserved.