Polymer networks were prepared from high molecular weight branched poly(oxyethylene)s (POE) with di(oxyethylene) units and allyloxymethyl groups as side segments, and their ionic conductivity and mechanical properties were investigated from the viewpoint of polymer solid electrolyte. The optimal crosslinking brought about both high ionic conductivity comparable to the non-crosslinked high molecular weight branched poly(oxyethylene)s and good mechanical properties to give a better dimensional stability, when the network was doped with LiN(SO2CF3)(2) in the concentration of [Li]/[-O-] = 0.06. This observation implies that the control of network-chain density in the preparation of networks from the high molecular weight branched poly(oxyethylene) is useful for the material design of practical polymer solid electrolyte, as well as their composition, the kind of salt and its concentration.