Numerical investigation of the soot particle distribution in a conventional, direct-injection (DI) diesel flame was performed. An available experiment from a heavy-duty, DI diesel engine under conventional injection conditions was selected as a benchmark. This experiment reproduced the operating conditions of similar experimental studies performed on an optically accessible engine of same class at the Sandia National Laboratories. The KIVA-3V code was used in the simulation, which features an improved multi-step phenomenological (MSP) soot model, a KH/RT spray breakup model, an extended Zel’dovich NO formation model, a “Shell” ignition model, a laminar-and-turbulent characteristic time combustion (CTC) model, a RNG k- ε turbulence model, a piston-ring crevice model, a droplet wall impingement model, and a wall-temperature function heat transfer model. The prediction reinforces the diesel conceptual model put forth by Dec in many aspects, particularly regarding spatial correlations between the liquid-spray penetration, the fuel–vapor penetration, the flame structure, and the soot concentration distribution. It is observed that, while larger soot particles are located downstream in the fuel-rich “head vortex” zone, a thin layer of larger soot particles at a considerably lower mass concentration exists along the peripheral surfaces of the flame. This thin, soot particle layer might be a result of competition between surface growth and surface oxidation when younger particles flow outwards from the interior zone of combustion to the high-temperature surface of the flame plume. The details of soot-relevant quantities (e.g., particle size, number density, generic precursor, and acetylene concentrations) and reaction rates (e.g., soot inception, particle coagulation, surface growth, and oxidation) provide valuable insights into diesel soot formation and oxidation processes.