The microdomain morphology of a series of MDI/BDO/PTMG segmented polyurethane block copolymers has been observed by transmission electron microscope. The microdomain size ranging from several tens to a few hundreds A is in the same order with that measured from SAXS. The degree of the microdomain size dispersion increases with increasing the hard segment content in the copolymers. These observed microdomains can be considered as the basic morphology of the hard segment domains and the variation of their sizes and shapes can be related to the content and length distribution of the hard segments. The microdomains exhibit the tendency to aggregate nad form the macrodomains which are larger in size (usually from several thousands A to a few μm)and usually were mistaken for the basic morphology of the hard-segment domains in previous publications.