Lithium ion conducting solid polymer electrolyte based on carboxymethyl cellulose (CMC) complexed with lithium tetrafluoroborate was prepared by using solution cast technique. Ionic conductivity was measured using ac impedance analyzer and observed as 8.2 x 10(-6) S cm(-1) at room temperature. For further enhancement in the conductivity, plasticizer was introduced and it reached up to 3.7 x 10(-3) S cm(-1) at room temperature. Structural characterization was performed by using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscope technique. XRD results show the amorphous nature of the electrolyte film. FTIR spectroscopic analysis confirmed the complexation of the salt and plasticizer with the polymer matrix. Differential scanning calorimetry was used to determine the glass transition and melting temperatures of pure CMC and CMC-based polymer electrolyte film.