Abstract The interparticle distance between black aggregates in rubber was estimated by Fraunhofer diffraction with the mask of an electron micrograph of filled rubber. Up to 40 phr, the distribution of interpartiele distance has a sharp peak centered at 8000 A˚ with 20 phr and at 7000 A˚ with 40 phr. Above 60 phr, the interparticle distance distributes broadly in the vicinity of 4000 A˚. The size of black aggregates at 20 phr extends from 300 A˚ (one particle) to 3000 A˚, and the average diameter is 1200 A˚. Above 40 phr, black trends to aggregate more than at 20 phr, and the average diameter is about 1500–1600 A˚, the maximum diameter being 5000 A˚. In one black aggregate, there are about 40 spherical particles. The relation between the size and interparticle distance shows that isolated black aggregates are distributed in rubber under 40 phr and that at 40 phr they begin to connect partly with each other. Above 60 phr, a network of chains of black develops everywhere. The contact of black aggregates at 40 phr level corresponds to the abrupt change of physical properties of filled rubber.