Fe 3O 4 nanocomposite of PEO: LiClO 4 solid polymer electrolyte films have been prepared by means of solution casting method. FT - IR and X - RD methods have been employed to study structural changes and coordination within this Fe 3O 4 of PEO: LiClO 4 nanocomposite polymer electrolyte. The COC vibrational mode observed in pure PEO around - 1100 cm -1 is found to be broadened in its width accompanying shift to higher wave number in nanocomposite of PEO: LiClO 4: Fe 3O 4 electrolyte when compared to Pure PEO, and PEO: LiClO 4 films. The two major ClO 4 - species with different conformations were found in PEO: LiClO 4 electrolyte; one being free ClO 4 - ions with maxima at - 623 cm -1 and other ClO 4 - anion associates with the Li+ cation at ~ 635 cm-1 ((Li +...ClO 4 - (i.e., ion-pairing)). The intensity of ionpair formation is found to decrease with increasing of nanosized Fe 3O 4 composition to PEO: LiClO 4. This could be the direct indication of a higher degree of LiClO 4 salt dissociation by the addition of nanosized Fe 3O 4 to PEO: LiClO 4 solid polymer electrolyte. Numerous interactions are possible in the nanocomposite PEO electrolyte, where nano Fe 3O 4 surface has a Lewis acidic group capable of interaction with Lewis base centers of PEO polymer. Thus, this nanosized Fe 3O 4 oxide may acts as physical cross-linking centers for the PEO chains, and also facilitated salt dissociation, where negatively charged oxygen of Fe 3O 4 interact with lithium ion.