The main objective of this study is to determine the effect of thermal cracking on asphaltene molecular structure. Hence, n-heptane insoluble fractions were precipitated and collected from three different vacuum residues and their thermally decomposed products. The X-ray diffraction (XRD) analysis provided an understanding of the macro-structure of the asphaltene molecule and revealed the impact of thermal stress on its structural parameters. The XRD analysis indicated that the layer distance between the aromatic sheets (d(m)) and the distance between aliphatic chains and naphthenic sheets (d(gamma)) were not substantially affected by the cracking severity. Furthermore, the average diameter of the aromatic sheet (L-a) has not changed much with thermal cracking. This finding was associated with a constant number of aromatic rings in the aromatic sheet (NOar). The cluster diameter (L-c), on the other hand, decreased in size as the severity of cracking increased. The decrease in cluster diameter is due to the loss of aliphatic carbon in the side chains as well as the decrease in the number of aromatic sheet per stack (M). (C) 2015 Elsevier Ltd. All rights reserved.