The aggregation behavior of solute molecules in under- and supersaturated solutions was investigated using an organic compound p-acetanisidide (PAC) as a model compound and NMR method. The aggregation behavior was discussed by comparing the structure of aggregates with that of a single crystal determined by X-ray crystallographic analysis. And also, the relationship between the aggregation and the nucleation was discussed. PAC molecules were found in aggregates before nucleation, not only in the super-saturated solution but also in the under-saturated solution. The growth of aggregates in the under-saturated solution was slower than that in the super-saturated solution. The local structures observed in the aggregates were intermolecular hydrogen bonding between N-H hydrogen and C=O oxygen, and the intra- and intermolecular access of methoxy group hydrogen to (orto-) hydrogen on a benzene ring within a distance of 4 Angstrom. These interactions were also observed between adjacent PAC molecules in crystal. The whole structure of aggregates, however, was not the same as that of the crystal. The formation of aggregates before nucleation in the under-saturation condition suppressed the nucleation, and the formation of aggregates in the super-saturation condition accelerated the nucleation.