Recently, three-dimensional (3D) direct observations and analysis of polymer morphologies draw considerable attention because they not only provide intuitive understanding of the morphologies but also offer novel structural parameters that can never be obtained by other technique. The most sophisticated transmission electron microtomography (TEMT) achieves spatial resolution of less than 1 nm in the 3D reconstructed images. In the present paper, applications of TMET to polymer nanostructures, that is microphase-separated structures of block copolymers (Gyroid-structures), were reported in order to demonstrate huge and promising capabilities of the new technique.