Among the naturally occurring gluco-disaccharides, trehalose has the highest glass transition temperature (Tg) and the largest activation energy of enthalpy relaxation in the glassy state. Here, to understand why trehalose has such unique properties, we carried out FT-IR measurements and molecular dynamics simulations for trehalose and neotrehalose, and analyzed the hydrogen bond properties of both sugars in the amorphous states. By comparing the results for trehalose and neotrehalose, we reached the following interpretation: in the trehalose glassy matrix sugar molecules would be homogeneously distributed, whereas in the neotrehalose glassy matrix sugar molecules tend to be distributed separately into densely packed regions and sparsely packed ones. It is concluded that in the trehalose glassy matrix the conformational uniformity of sugar molecules brings about better (homogeneous) molecular packing, which in turn leads to the high Tg and the stability of the glassy state. Ultimately, such a unique property of trehalose originates from the presence of α, α-1, 1 glycosidic bond responsible for the conformational rigidity.