Water soluble polyacrylic acid(PAA) was modified by alkylation reaction. The resulting hydrophobically modified sodium polyacrylates(HMPAANa) with various molecular weights were prepared. We studied the hydrophobic aggregation of HMPAANa polymers. The optimum hydrophobic aggregation condition was obtained at a proper ionic strength of medium. In an aqueous media, optimum hydrophobic aggregation condition was obtained when HMPAANa was neutralized to 20%. At a fixed ionic strength the aggregation could be classified to inter- and intra-molecular aggregation with respect to the polymer concentration. When the solution was in a semi-dilute regime, it became a gel on account of the intermolecular aggregation. The polymer of larger M. W. became a gel at a lower concentration for the samples of different M. W's with the same degree of modification. The M. W. dependence of the gelation was very similar to the M. W. dependence of C^* of the snailing theory. Below the concentration of gelation, several polymers aggregated and became a very large particle. It could be observed by gel filtration chromatography technique. In a dilute regime, HMPAANa polymers aggregated infra - molecularly. So they showed the contracted hydrodynamic volume.