  For the first time, Kataoka \& Stawarz reported a clear detection of a hard
X-ray excess, above $\gtrsim$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
$\gamma_{\rm min}\approx19$, 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.
