Transmission electron microscopy (TEM) was applied to characterize the microstructure of Tc metal and technetium dioxide synthesized by the decomposition of NH4TcO4. The morphology of the Tc metal and well-resolved TcO2 particles were characterized using bright-field TEM and scanning TEM modes. Structural characterization of these two samples using high-resolution (HR) TEM was successfully performed for the first time on the nanoscale. The morphology of Tc metal showed significant differences when compared to TcO2. The crystal structure of Tc metal on the nanoscale was shown to contain well-resolved lattice fringes without any defects. Because of the deficiency in point resolution, however, the two-dimensional structure details of Tc metal could not be observed as expected. On the other hand, structural details of TcO2 were prominent at high resolution. With a 2-fold multiplicity in both directions, TCO2 showed a unique atomic distribution corresponding to a monoclinic unit cell. Furthermore, the lattice parameters of the samples were refined by the Rietveld analysis of the powder X-ray diffraction patterns and were estimated by HRTEM. In the case of technetium dioxide, the stoichiometry was approximated to be TcO2.3 using quantitative analysis of X-ray energy dispersive spectrometry. Electron energy-loss spectrometry verified the chemical phase of the two samples by their different chemical environments based on an energy shift of 2.0 eV of the N-23 edge between TcO2 and Tc metal.