Asphaltene aggregation causes several problems in the petroleum industry, and an understanding of the aggregation behavior is needed to solve the problems. In this study, the effects of asphaltene concentration on aggregate size were investigated to elucidate the aggregation mechanism of asphaltene. Diffusion coefficients (D) of three asphaltenes from the vacuum residue (VR) of Khafji, Iranian Light, and Maya crude oils and one resin were determined in deuterated chloroform solution using pulsed-field gradient spin-echo H-1 NMR. The pulsed-field gradient spin-echo 1H NMR of asphaltenes and resin showed different diffusion behaviors from well-characterized reference compounds such as polystyrene because of structural and compositional irregularities; that is, they are a complex mixture of molecules of various molecular weights and structures. From the D values, their average hydrodynamic radii were estimated. The concentration dependency (0.1 - 30 g/L) of the D values and the hydrodynamic radii support the widely accepted stepwise aggregation mechanism (i.e., monomer < small aggregates < medium-size aggregates < large aggregates (precipitate)). At low concentrations (0. 1 and I g/L), the D values corresponding to the range of small to medium aggregates were observed, and at higher concentrations (10 and 30 g/L), only medium aggregates were detected. Similar D values were obtained for the three asphaltenes, although Maya asphaltene gave slightly lower D values. For the resin, higher D values (lower hydrodynamic radii) were obtained than the asphaltenes.