  We study the peak energy ($E_{\rm{p}}$) distribution of the $\nu F_{\nu}$
spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57
bursts observed by {\em High Energy Transient Explorer 2} ({\em HETE-2}) French
Gamma Telescope and discuss its implications for the jet structure models.
Combining the observed $E_{\rm{p}}$ distribution of {\em HETE--2} GRBs/XRFs
with that of BATSE GRBs, we find that the observed $E_{\rm{p}}$ distribution of
GRBs/XRFs is a bimodal one with peaks of $\lesssim 30$ keV and $\sim 160-250$
keV. According to the recently-discovered equivalent-isotropic
energy-$E_{\rm{p}}$ relationship, such a bimodal distribution implies a
two-component structure of GRB/XRF jets. A simple simulation analysis shows
that this structured jet model does roughly reproduce a bimodal distribution
with peaks of $\sim 15$ and $\sim 200$ keV. We argue that future observations
of the peak of $\sim 15$ keV in the $E_{\rm{p}}$ distribution would be evidence
supporting this model. {\em Swift}, which covers an energy band of 0.2--150
keV, is expected to provide a key test for our results.
