Carbon nanofibres aerosolized by the agitation of as-produced commercial powder have been characterized in situ by using the differential mobility analyser-aerosol particle mass analyser (DMA-APM) method to determine their structural properties such as the effective density and fractal dimension for toxicology study. The effective density of the aerosolized carbon nanofibres decreased from 1.2 to 0.4 g cm-3 as the mobility diameters increased from 100 to 700 nm, indicating that the carbon nanofibres had open structures with an overall void that increased with increasing diameter, due to increased agglomeration of the nanofibres. This was confirmed by transmission electron microscopy (TEM) observation, showing that 100 nm mobility diameter nanofibres were predominantly single fibres, while doubly or triply attached fibres were seen at mobility diameters of 200 and 400 nm. Effective densities calculated using Cox's theory were in reasonable agreement with experimental values. The mass fractal dimension of the carbon nanofibres was found to be 2.38 over the size range measured and higher than that of single-walled carbon nanotubes (SWCNTs), suggesting that the carbon nanofibres have more compact structure than SWCNTs.