[en] This paper reports on a study of microstructure of Nb/AlO_x---Al/Nb Josephson junctions by cross-sectional transmission electron microscopy (TEM) which yielded much information regarding the unction barrier region. Both thick Nb and several-nm Al form polycrystalline films with columnar structures. Nb is oriented to the (110) plane, and Al is (111). The 200-nm lower Nb has a wavy surface with ∼5 nm smoothness, but its surface is planarized by several-nm Al deposited on it. Thus AlO_x with a smoothness under 1 nm can be formed on Al. The upper Nb has a good crystalline structure even just above the AlO_x barrier