Crude oil contains heavy fractions such as asphaltenes, resines and maltenes, which are a solubility class and not a specific boiling range. Most of problems associated with the heavy oil refining process are attributed to the presence of asphaltenes. Asphaltenes are thought to be the aggregates of polyaromatics with hetero atoms and metals, and believed to be the precursors of coke formed during refining process. In this study, changes in the aggregated structure of asphaltene induced by pretreatment with solvents were investigated in terms of molecular mobility obtained with pulse ^1H-NMR. The results obtained using pulse ^1H-NMR demonstrate that the solvent pretreatments are effective to dissociate the aggregated structure of asphaltenes. It is also clarified that the dissociation behaviour of asphaltene aggregates is able to characterize by means of pulse ^1H-NMR.