In the paper, results of microscopic analyses of the weld metal taken from weld joint of the thick-walled austenitic stainless steel pipeline of the primary circuit of the nuclear power plant WWER-440 are presented. The weld joint has been exposed in the main circulation pipeline approximately 180,000 h at temperatures approaching 300• C. A sample taken from a top part of the multi-pass weld joint has been analyzed by scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and high-resolution transmission electron microscopy. The analyzed weld metal exhibited a duplex microstructure consisting of the austenite phase and a semi-continual network of δ-ferrite phase. The mean hardness of the weld metal was 195 HV1. In the weld metal, globular non-metallic inclusions, as well as carbonitride particles of the MX type and chromium based carbide particles of M23C6 type, were identified. At the phase interfaces of δ-ferrite/austenite, the presence of particles of σ-phase placed in the volumes of δ-ferrite network was detected. The presence of σ-phase and M23C6 carbide particles in the top part of the weld can be explained as a consequence of the short-term expositions of the weld metal in the temperature range from about 900 to 600• C during the multi-pass welding process of the primary circuit of the nuclear power plant. High--resolution transmission electron microscopy analysis of the ferrite phase regions revealed in thin foils the presence of mottled contrast indicating the spinodal decomposition of the δ-ferrite to Cr-rich α -and Fe-rich α-phases. It is supposed that the decomposition of the δ-ferrite phase started during final stages of the multi-pass welding process at thermal expositions around 475• C and continued during long-term exposition at the operation temperatures of the primary circuit.K e y w o r d s : stainless steel, weld metal, low-temperature thermal aging, σ-phase, carbide phase, α -phase