A new cross-linker, poly[siloxane-(g-oligo(ethylene oxide)-co-acrylate)], was synthesized and used to prepare the solid polymer electrolytes (SPE) by in situ thermal curing method with the addition of the ion-conducting plasticizers such as low molecular weight poly(ethylene oxide)dimethyl ether (PEGDME) and poly(siloxane-g-ethylene oxide) (PSi-PEG). Increase of conductivity and decrease of T g were observed as increasing the content of plasticizer. The ionic conductivity was measured to be 7.13×10−4 Scm−1 with 70wt.% PEGDME and 4.18×10−5 Scm−1 with 70wt.% of PSi-PEG at 30°C. The electrochemical stability window of the resulting solid polymer electrolyte could be extended to up to 4.5V and 5.2V for PEGDME and PSi-PEG, respectively. Thermal stability of polymer electrolyte was greatly enhanced with siloxane based plasticizer. The degradation of SPE with PSi-PEG started at ca. 350°C. The SPE plasticized with PSi-PEG, which has good electrochemical stability and thermal stability, could be a promising solid polymer electrolyte for lithium polymer batteries.