A Polyaniline/Manganese dioxide (PANI/MnO2) nanocomposite contains five weight percentages of MnO2. It has been successfully prepared by in situ polymerization. The structural, optical, and CONDUCTIVITY of nanocomposites remain relative to changes with respect to the weight percentage of MnO2. The structural, morphological, optical, and electrical CONDUCTIVITY investigation of pure MnO2, pure PANI, and nanocomposites were done with powder XRD, HRTEM, SEM, FT-IR, UV and Impedance spectra. XRD results of the PANI/ MnO2 nanocomposites say that the crystalline structure is converted into a very less crystalline structure due to the incorporation of MnO2 which is inside PANI chain. The HRTEM and SEM images are confirmed to the nanocomposite formation, morphology studies and also supported to the XRD results. From the optical spectra, MnO2 nanoparticles have been impressed in the surface of PANI. It works as the compensator in the formation of nanocomposites. UV spectral analysis reveals that the absorption of MnO2 modifies the absorption wavelength of under visible light in whole range. The absorption wavelength of nanocomposites is 288 nm and 337 nm. AC electrical CONDUCTIVITY of the prepared nanocomposites from impedance spectroscopy was carried out and compared with pure materials. The AC CONDUCTIVITY of as-prepared nanocomposites has been analyzed in the range of 298 K to 423 K. The AC CONDUCTIVITY of nanocomposites varies depending upon a change of logarithmic frequency.