Composite polymer electrolytes based 49% poly(methyl methacrylate)-grafted natural rubber (MG49) incorporating lithium triflate (LiCF3SO3) were prepared. The study mainly focuses on the ionic conductivity performances and mechanical properties. Prior to that, carboxymethyl cellulose was synthesized from kenaf fiber. The films were characterized by electrochemical impedance (EIS) spectroscopy, linear sweep voltammetry (LSV), universal testing machine and scanning electron microscopy (SEM). The conductivity was found to increase with carboxymethyl cellulose loading. The highest conductivity value achieved was 6.5 × 10-6 Scm-1 upon addition of 6 wt% carboxymethyl cellulose. LSV graph shows the stability of this film was extended to 2.7 V at room temperature. The composition with 6 wt% carboxymethyl cellulose composition showed the highest tensile strength value of 7.9 MPa and 273 MPa of Young’s modulus. The morphology of the electrolytes showed a smooth surface of films after addition of salt and filler indicating amorphous phase in electrolytes system. Excellent mechanical properties and good ionic conductivity are obtained, enlightening that the film is suitable for future applications as thin solid polymer electrolytes in lithium batteries.