Crystallization kinetics of polypropylene (PP) nucleated with sorbitol-type nucleating agents was studied with fast differential scanning calorimety (FDSC) at high supercooling and correlated to the optical characteristics of mold injected parts. FDSC was used for determination of the critical cooling rate for occurrence of non-isothermal crystallization and the time to maximum crystallization rate during isothermal crystallization after rapid cooling. The nucleating agents studied were 1,3: 2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN). In contrast to particulate nucleating agents, where the number of particles determines the number of heterogeneous nuclei, sorbitol-type nucleating agents self-assemble a nano-fibrillar super-structure by phase separation of sorbitol-type molecules that were previously dissolved in the polymer melt. An approach is proposed to describe the efficiency of nucleating agents self-assembly. In combination with haze data two different crystallization mechanisms are identified, which are interpreted as three-dimensional spherulitic crystallization at high haze and one-dimensional (rod or shish-kebab-like) crystallization at low haze.