  Deep observation of the High Energy Stereoscopic System (HESS) on the most
extended pulsar wind nebula HESS J1825-137 reveals an enhanced energy-dependent
morphology, providing useful information on the particle transport mechanism in
the nebula. We find that the energy-dependent morphology is consistent with a
diffusion-dominated transport of electrons/positrons. It provides an
alternative possible interpretation for the unusually large spatial extent
(i.e., $\gtrsim 100$pc) of the nebula, which could then be attributed to the
diffusion of escaping electrons/positrons from a compact plerion. The influence
of various model parameters on the energy-dependent extent of the nebula is
studied in the diffusion-dominated scenario. We also show that the
energy-dependent morphology of the nebula may also be used to study the
spin-down history of the pulsar.
