Using a large sample of blazars of the Fermi observations presented by Abdo et al.,we constructed a sample of blazars including high energy peaked BL Lac objects (HBLs),low energy peaked BL Lac objects (LBLs) and flat-spectral radio quasars (FSRQs).These unique characteristics make it possible to unambiguously address the question of how HBLs,LBLs and FSRQs are related.In this paper,we investigated the relationship between X-ray and γ-ray spectral indices (α x-αγ),as well as the relationship between the broadband spectral indices (α ro-α rx,α ro-α ox,α ro-α xγ and α rx-α xγ) for this sample.The color-color diagram shows that there is a significant correlation between both quantities when all three subclasses of blazars are considered,which suggests that there is a unified scheme for blazars.On the other hand,the α x-αγ diagram reveals that three kinds of blazars have different spectral energy distributions:the trend of HBLs is different from that of FSRQs and LBLs,whereas FSRQs and LBLs have a similar trend,which hints that FSRQs and LBLs have similar spectral properties,but HBLs have distinct spectral properties.In addition,the broadband energy distributions also reveal the similar spectral properties with that of the α x-αγ diagram.The spectral properties revealed from the Fermi sample do not support the blazar sequence reported by Fossati et al.and Ghisellini et al.
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