Structure and properties of high molecular weight comb-shaped poly(oxyethylene)s with tri(oxyethylene) segments as side chains (TECs) were evaluated as a polymer solid electrolyte. The weight-average molecular weights of the TECs were in the order of 106, and the contents of the side chain were 5, 11 and 18 mol%. The presence of tri(oxyethylene) side chains decreased the crystallinity of oxyethylene segments, i.e., TECs with higher side chain content showed lower crystallinity. The crystallite size of oxyethylene units in TEC films did not change, but the amorphous phase content became higher by the introduction of tri(oxyethylene) side chains. The crystalline structure in TECs was found to be Form I monoclinic system and a 7/2 helical structure. When LiClO4 was doped in the TECs at the concentration of [Li]/[O]=0.05, TECs with 11 and 18 mol% tri(oxyethylene) segments showed the ionic conductivities in the order of 10−4 S cm−1 at 30°C and 10−3 S cm−1 at 80°C, respectively, which were a class of the highest ionic conductivity reported. TECs doped with the salt formed very elastic self-standing film without crosslinking. Salt-doping at the concentration of [Li]/[O]=0.05 decreased the crystallinity of polymers, but the crystallite sizes were almost equal with those of the non-doped films.