The nanoscale mono- (powders) and complex (compacted nanocomposites) carbides of $d$-transition metals are synthesized by mechanical alloying in a high-energy planetary ball mill from a charge containing the carbon nanotubes. The effect of multiwalled carbon nanotubes on reaction milling of the obtained materials is analyzed. The features of formation mechanism of metal carbides at the mechanical alloying are clarified. Particularly, as is shown, at the first stage of the synthesis (up to 60 min of processing of the charge in a ball mill), the amorphization of the carbon nanotubes and crushing of particles of the initial metal along the grain boundaries occurs concurrently. Then, the amorphous carbon enters into the metal lattice forming an interstitial solid solution, resulting in deformation of the metal crystal lattice. At the second stage of synthesis (from 60 to 250 minutes of processing), the process of embedding of the carbon atoms in metal matrix is accelerated and the formation of the carbide phases on surface of the parent metal particles begins. The third stage of synthesis completes the formation of carbide. As revealed, the processing time required for the complete transformation of the initial components to the carbide correlates with the enthalpy of its formation, and the fields of mechanical stress are relaxed over two main channels: heating and grinding. As found out, the carbides of $d$-transition metals studied in this work are formed mainly due to self-supporting reaction at milling. The efficiency of using carbon nanotubes in the fabrication of nanocomposite materials with improved functional characteristics is shown. As revealed, the reaction milling is effective for the synthesis of multicomponent carbides (substitutional solid solutions).