In this paper, we report for the first time a microstructural characterization of pyrolysis oils obtained from biomass. Bio crude oils (BCOs) are good candidates as substitutes for mineral oils as fuels. By using small-angle neutron scattering (SANS), we show that BCOs are nanostructured fluids constituted by a complex continuous phase and nanoparticles mainly formed by the association of units of pyrolytic lignins. The aggregation of these units during the time produces branched structures with fractal dimension D_f between 1.4 and 1.5, which are responsible for BCO aging. SANS results fully support the recently formulated thermal ejection theory, accounting for the mechanism of formation of the lignin fraction in oils obtained from fast pyrolysis of biomass.