Nanoparticles are considered promising additives to diesel fuel and significantly affect engine performance and exhaust emission levels. Nitrogen-doped multiwalled carbon nanotubes (N-doped MWCNTs) are one of the effective catalysts owing to their promising properties, which have recently increased their usability in many applications. These features make them good candidates as a catalyst to enhance diesel engine performance. The effects of adding N-doped MWCNTs into diesel fuel for a single-cylinder CI engine working at a rated speed of 1500 rpm and different loads were studied. Four doses of N-doped MWCNTs were added to diesel fuel using ultrasonic dispersion. Multiwalled carbon nanotubes (MWCNTs) were used as reference additives to compare their results with those obtained by N-doped MWCNTs. The results illustrated that most blends of N-doped MWCNTs were preferred in combustion behavior, engine performance, and exhaust emission analysis with respect to diesel and MWCNTs blends. Thus, there was a significant increase in the in-cylinder pressure and heat release rate compared to pure diesel. Also, the brake thermal efficiency was increased, and the brake specific fuel consumption was reduced compared to diesel. For all N-doped MWCNTs blends, there were remarkable reductions in NOx, soot, and CO formations, and it is mainly preferred over MWCNTs/diesel blends in exhaust emissions reduction.