Peptide-assembled hydrogels have emerged as a promising biomaterial for tissue engineering due to their controllable structures as well as biocompatible and biodegradable characteristics. However, the detailed situations of nanofibrillar mechanics during the formation of peptide-assembled hydrogels remain poorly understood. The advent of atomic force microscopy (AFM) provides a powerful tool for nanoscale characterizations and measurements of native biomaterials without pretreatments. In this paper, AFM peak force tapping multiparametric imaging was utilized to investigate the morphology and mechanics of individual nanofibrils during the gelation and degradation process of peptide-assembly hydrogels. Nanofibrillar hydrogels were fabricated by the self-assembly reactions of phenylalanine peptide molecules. AFM topography imaging distinctly visualized the diverse assembly behaviors of peptide-formed nanofibrils during the gelation and degradation of hydrogels, which were significantly correlated with the changes of nanofibrillar mechanics (Young's modulus, adhesion force, deformation, and persistence length) revealed by AFM mechanical imaging. The research provides a novel idea for nanoscale imaging and mechanical analysis of nanofibrillar behaviors in peptide-assembled hydrogels, which will have potential impacts on the characterization and design of supramolecular biomaterials.