A thermodynamic model is presented to account for the nongraphitizing behavior of small (<200 A˚) carbon black particles. Only pure carbon is considered, hence no compositional effects are included in these calculations. The ungraphitized carbon black particle is modeled as concentric turbostratic spheres of graphene layers. The partially graphitized particle is modeled to consist of a graphitic dodecahedral shell and a spherical turbostratic core. Free‐energy differences are calculated between a fully turbostratic particle and a partially graphitized particle. The temperature dependence of the free‐energy difference has not been included in these calculations. It is found that the effective activation barrier to graphitization increases with decreasing particle size. The concepts developed in this model are extended to explain the nongraphitizing behavior of all hard carbons.