The microstructure of the heat affected zone after welding was investigated in Mod.9Cr–1Mo steel, using TEM and STEM-EDX. The microstructure of thin foil was observed at the fusion line, and at the positions of 0.5mm, 1.0mm, 1.5mm, 2.0mm, 2.5mm, 3.0mm and 3.5mm to the base metal side of the fusion line. Martensite structure with very fine lath and high dislocation density was confirmed at all positions. Twins with a twin plane of (112) were locally observed at all positions. Elemental mapping was obtained for all positions by means of STEM-EDX. Inclusions of mainly Si were formed at the fusion line but not at the other positions. No precipitates could be detected at the fusion line or at the position of 0.5mm. On the other hand, MX particles were observed at the positions of 1.0mm, 1.5mm, 2.0mm, 2.5mm, 3.0mm and 3.5mm even after welding. M23C6 particles were also confirmed at the positions of 2.0mm, 2.5mm, 3.0mm and 3.5mm. Very fine equiaxed grains were locally observed at the positions of 2.0mm and 2.5mm. The Cr content of the equiaxed grains was about 12mass%, although the martensite area included about 8mass% Cr.