The development of technical strategies for synthesizing nanodiamond (ND) under moderate conditions is still an attractive and challenging issue. Herein, an attempt has been made to investigate the effect of the Fe-introduction mode on the catalytic synthesis of ND during the chlorination of vanadium carbide (VC) at ambient pressure. The results show that when Fe powder as a catalyst is adhered on the surface of the VC particle, the resultant carbide-derived carbon (CDC) by chlorinating VC at 800 °C behaves as a remarkably higher structural ordering. However, as the Fe powder is enclosed within the interior of VC particles, the structural ordering of the CDC obtained at the same chlorination temperature becomes greatly lower, and more interestingly, ND structures can be observed in the prepared CDC. This suggests that the catalytic synthesis of ND during VC chlorination at ambient pressure is heavily dependent upon the Fe-introduction mode.