For the first time, Kataoka & Stawarz reported. a clear detection of a hard X-ray excess, above greater than or similar to 20 keV, in the high-synchrotron-peaked BL Lac object. Mrk 421. We find that this feature may. not be. produced by the low-energy part of the. same electron population that. produced the Fermi/LAT gamma-ray. Because of that, it is. required. that the power-law electron energy go. down to gamma(min) approximate to 19, which predicts a very strong radio emission (radio flux larger than the observed), even considering the synchrotron self-absorption effect. We investigate the possibility of this excess being. produced from the spine/layer jet structure, which has been clearly detected in Mrk 421. We find that. (1) similar to one-zone modeling, the spine emissions provide good. modeling of the. broadband spectral energy distribution, except for. the hard X-ray excess; and. (2) the hard X-ray excess can be well represented by the synchrotron photons (from the. layer) being inverse Compton scattered by the spine electrons.