Cross-linked solid polymer electrolytes (cross-linked SPEs) composed of poly(ethylene glycol) dimethyl ethers (PEGDMEs) with various molecular weights and high molecular weight poly(ethylene oxide) (PEO), Acryl-HBP having various ethylene oxide (EO) units (3EO (n = 3), 4EO (n = 4), 6EO (n = 6), 11EO (n = 11)), and LiN(SO2CF3)2 salt were prepared by using radical initiator and electron beam (EB) irradiation. The effects of molecular weights of PEGDMEs and PEO, EO units in the Acryl-HBP, crosslinking methods, and EB irradiation intensity on the ionic conductivity and mechanical property for the cross-linked SPEs were investigated. Ionic conductivities of the cross-linked SPEs composed of Acryl-HBP having 4EO (n = 4) units and PEGDME with molecular weight of 500 were found to be 1.24 × 10−4 S/cm at 20 °C and 1.97 × 10−3 S/cm at 80 °C for benzoyl peroxide radical initiator and to be 2.44 × 10−4 S/cm at 20 °C and 3.22 × 10−3 S/cm at 80 °C for EB irradiation, respectively. Further addition of a copolymer (EO-AGE) of ethylene oxide with allyl glycidyl ether improved mechanical properties of the cross-linked SPE, and provided mechanical strength suitable for practical lithium battery application. The Li4Ti5O12/cross-linked SPE/LiFePO4–C cell was assembled by using the cross-linked SPE modified with EO-AGE, and the battery performance was evaluated. The battery showed reversible charge-discharge capacity of 42 mAh at 25 °C and C/10 rate.