After the third Chinese nuclear bomb test, radioactive aerosol was collected on fiber filters from the atmosphere at different altitudes. By comparing the measured β−‐decay curves with the theoretical β−‐decay curves the fractionation factor of the fresh fission products and the activity of 237U in the samples were determined. Furthermore, the samples were autoradiographed and the spot‐size distribution and background blackening of the autoradiographs were measured. By evaluating the particle sizes (particle activities) from spot sizes and by comparing the results of β−‐decay curve analysis and of the autoradiographs it is shown that there is a relation between fractionation degree and particle‐size distribution; nuclides originating from volatile precursors are enriched in small particles, and nuclides from refractory precursors are enriched in large particles. Uranium 237 is also enriched in small particles and fractionates like nuclides descending from volatile precursors.