Small-Angle X-ray Scattering (SAXS) technique was used to characterize the structure of Asphaltene aggregates as a function of its concentration in Toluene solutions. Asphaltene concentration ranged from 0.01 to 1% w/v. Solid Asphaltenes were obtained from a Colombian heavy crude oil following the ASTM D6560-12 standard method by precipitation with n- heptane. Structural properties were interpreted in terms of the radius of gyration (Rg) and the power-law exponents (d). The obtained spectra for Asphaltene-Toluene systems allowed  us to explore two regions. At low values of the scattering vector q (q < 3 x 10-2), we found that Asphaltenes self-associate to form aggregates with fractal properties. These structures exhibit a particular transition from surface fractals to mass fractals as concentration increased. In the second one (q > 3 x 10-2), scattering curves showed that Asphaltenes form small dense regions, in which the intensity of the scattered radiation rises with increasing concentration. The observed results demonstrate that Asphaltenes in Toluene solutions  coexist as large scale aggregates and nanoaggregates, likewise these structures are conditioned by the Asphaltene concentration and self-association processes.