Mapping the structure evolution and mechanical properties of elastic polymers or biomaterials during bulk deformation has been difficult, however, this information have long been thought to be a key to understanding the structure-mechanical property relationships and will help guide the design of new materials for desired properties or uptake. Here we use a nanomechanical mapping to assess the structural evolution and mechanical properties of a deformed isoprene rubber (IR) to elucidate a self-reinforcement mechanism found in this material. A hierarchical nanofibrillar structure, ranging from several to a hundred nanometers of fibers oriented parallel to the stretching direction was found in the deformed IR. The nanofibers, connected by oriented amorphous tie chains, form a network structure that is responsible for significantly enhanced stress, a key factor giving rise to the self-reinforcement of IR and, more than likely, most elastomers that undergo strained-induced crystallization.