It is often difficult to observe nanoscale structures of polymeric materials using conventional transmission electron microscopy (TEM) because of their weak scattering contrast. To produce quantitative image contrast without any staining, low-angle annular dark-field scanning transmission electron microscopy (LAADF-STEM) was studied for its applicability for observing fine structures in block copolymers. The LAADF-STEM images displayed microphase-separated morphologies of block copolymers with high S/N contrast depending on the intrinsic density difference because of nonstaining. We found that the temperature dependence of the image contrast showed a kink around the glass-transition of the constituent phase, from which one can estimate glass transition temperatures and thermal expansion coefficients at nanoscale. This indicates that the LAADF-STEM imaging is an effective tool to quantitatively image nanoscale phases of polymers.