An atom-based aggregation structure for oligomer-like polycarbosilane (PCS) was constructed by simulating multiple molecules composed of six-type silicon-carbon linear and ring units. A critical length was defined via the pair distribution function transferred from the wide-angle X-ray scattering curve and used to characterize the average particle size of PCS. The results revealed that the PCS molecules were untangled from each other, and every molecule formed an ellipsoidal-like particle. The average size of single-molecule particles predicted by the atom-based aggregation model was 1.4 nm for PCS with a number-average molecular weight of similar to 1200 g/mol, which was comparable with those evaluated on the basis of the synchrotron radiation small-angle X-ray scattering data for extremely diluted ethanol solutions consisting of PCS and the wide-angle X-ray scattering data for PCS powders. The single-molecule particles in PCS had a fractal type of mass, suggesting a microporous structure.