We demonstrate herein the variation in viscoelastic properties of supramolecular hydrogels (SMGs) composed of two amphiphiles, N-Palmitoyl-Gly-His (PalGH) and sodium palmitate (PalNa). PalGH molecules in water form lamellar-like assemblies, which stack into sheet-shaped aggregates, resulting in the evolution of three-dimensional network structures. Once PalNa is added to PalGH, the alkyl groups of PalNa incorporate themselves into the hydrophobic cores of PalGH lamellar-like assemblies, resulting in a change in the assembly from lamellar-like to fibrous micelle-like. Consequently, sheet-shaped aggregates turn into flexible fibrils, which form bundles, resulting in network structures. Mixed hydrogel network structures differ in morphology from those in homogenous PalGH and PalNa hydrogels. Changes in the network structure eventually alter the bulk viscoelastic properties of hydrogels. These results demonstrate that the viscoelastic properties of supramolecular hydrogels can be tuned by controlling the aggregation states.