The main objective of this work is to determine the effect of thermal cracking on asphaltene molecular structure by using nuclear magnetic resonance (NMR). The NMR is used to identify and quantify the structural parameters and the functional groups, which subsequently helped in deriving a hypothetical average structure for the asphaltene molecules. The NMR analysis revealed a number of valuable findings and developed a better understanding about the nature of structural changes that asphaltene molecules undergo during the thermal cracking of vacuum residues (VRs). For instance, the polyaromatic structure of asphaltene in the parent VRs consists of peri-condensed cores interlinked through aliphatic bridges. This peri-condensed polyaromatic core converts to cata-condensed polyaromatic as the reaction severity increases. The NMR analysis confirmed that the observed reduction in the average molecular weight of asphaltene molecules, as reaction severity increases, is mainly due to cracking of the saturated parts of the asphaltenes molecules, loss of sulfur and nitrogen located in the saturated parts, and shrinking in the aromatic core. Another observation is the growth in the percentages of aromatic carbon and aromatic carbon bearing hydrogen with the increase in cracking severity.