Effect of propylene carbonate (PC) concentration on the properties of a solid polymeric electrolyte containing poly(methyl methacrylate) PMMA, 49% of poly(methyl methacrylate)-grafted natural rubber (MG49) and lithium tetrafluoroborate (LiBF₄) salt has been studied. Solid polymeric electrolyte of PMMA-MG49-PC-LiBF₄ that was prepared by solution casting technique produced a freestanding film. FTIR analysis confirmed that an interaction occurs between lithium salt and oxygen atoms at carbonyl and ether groups in polymer host with the presence of plasticizer, PC. Morphology studies by SEM show that the addition of PC reduces the particles size of LiBF₄ salt and increases the dark surface of the electrolyte system. As a result, the surface of the electrolyte becomes smoother. This indicates that the addition of PC into PMMA-MG49-LiBF₄ have made the electrolyte system become more amorphous. The addition of PC enhanced the ionic conductivity of the electrolyte. It was observed that the conductivity of the electrolyte increases with PC concentration. The highest conductivity was obtained at 50 wt. % PC with the conductivity of 7.4×10^-6 Scm^-1. The ionic conductivity of PMMA-MG49-LiBF₄ without the plasticizer was found to be 2.7×10^-8 Scm^-1.