We study the peak energy (E_p) distribution of the ν F_ν spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by High Energy Transient Explorer 2 ( HETE-2) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed E_p distribution of HETE--2 GRBs/XRFs with that of BATSE GRBs, we find that the observed E_p distribution of GRBs/XRFs is a bimodal one with peaks of ≲ 30 keV and ∼ 160-250 keV. According to the

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... ecently-discovered equivalent-isotropic energy-E_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 ∼ 15 and ∼ 200 keV. We argue that future observations of the peak of ∼ 15 keV in the E_p distribution would be evidence supporting this model. Swift, which covers an energy band of 0.2--150 keV, is expected to provide a key test for our results.