Solid polymer electrolytes (SPEs) with high ionic conductivity and acceptable mechanical properties are of particular interest for increasing the performance of batteries. Our previous studies indicated that copolymers could be good candidates for SPE materials due to the variable properties contributed by each block. A series of copolymers applied in this research was poly(ethylene oxide)-block-polyethylene, PEO-b-PE, which contains a conductive block (PEO block) and a reinforcement block (PE block). This study examines the effects of composition and molecular weight of the copolymers on performance of the resulting SPEs. The ternary SPEs were prepared by addition of copolymers into PEO/LiClO4. It was found that increasing the PE block percentage in the copolymer resulted in a significant increase in both ionic conductivity and mechanical properties. The SPEs that contained the highest percentage of PE block, 80wt%, exhibits the best performances. The results showed an increase of more than two orders in ionic conductivity, about 350% increase in tensile modulus, and about 97% increase in ultimate tensile strength when the PE block increased from 50wt% to 80wt%. It was also observed that increasing the molecular weight of the copolymer resulted in better mechanical properties, and an identical ionic conductivity.