Structure of poly(oxyethylene) with tri(oxyethylene) side chains (TEC) doped with LiClO4 was quantitatively evaluated by using a wide-angle X-ray diffraction (WAY-D) method and the relation between the crystallinity of the branched polyether and the ionic conductivity was elucidated. By introducing tri(oxyethylene) side chains, the crystallinity of TEC became much lower than linear poly(oxyethylene) (PEO) and the ionic conductivity increased significantly. Further, the change of spherulitic structure was observed by using the polarized light microscope. The structure of the linear poly(oxyethylene) is the sheaflike spherulites and that of TEC is the spherulites which show the concentric extinction rings due to the influence of side chains. The structure of TEC doped with LiClO4 at the concentration of [Li]/[-O-] = 0.05 is the sheaflike spherulites again, but is different from that of poly(oxyethylene). The sizes of spherulites of TEC doped with LiClO4 are smaller and. the bundle of lamellae, which grew to the radial direction, is disordered and thicker than that of poly(oxyethylene). This is due to the presence of tri(oxyethylene) side chains and lithium salts between lamellae of poly(oxyethylene). From the oriented crystallization by uniaxial drawing or shearing, a "stacked lamellae" structure parallel to oriented direction is obtained. The structure may afford different ionic conductivities according to the direction of a material. In other words, anisotropic conductivity may be observed on this oriented material.