This paper examines the creep properties of modified 9–12% Cr steels, especially the P91 grade. This steel is widely-used in many power plant constructions all over the world. Welding P91 steel components is one of the most important technological procedures. The aim of the investigation performed is to why creep rupture on welded joints at high temperatures is often observed in the heat-affected zone (HAZ), especially in the so-called inter-critical zone (IC-HAZ). The creep rupture strength (CRS) of cross-weld joints is usually about 20 to 30% lower than that of the base material, in the case of creep-resistant modified chromium steels. Precipitation strengthening (PS) depends on the inter-particle spacing (IPS) of secondary phases — MX nanoparticles and/or M23C6 particles. With increasing IPS of secondary phases, the proof stress and creep rupture strength decreases, while creep rate increases. With respect to the small size of each in the HAZ region and to determine the IPS in the HAZ, the simulation technique was used for estimation of the microstructure and properties. The Smitweld machine was used to reproduce the thermal cycles in the HAZ. The microstructure of the HAZ-simulated regions was analysed by transmission electron microscopy. Real creep tests were carried out on specimens with HAZ-simulated regions and also on the P91 real welded joints. It was found that inter-particle spacing of secondary phases in the inter-critical region of the HAZ is much higher than that of the base material. Therefore, the proof stress and hardness, as well as creep rupture strength of the HAZ are lower than that of the base material. Creep test results of the real welded joints and simulated HAZ regions are in good correlation with all theoretical presumptions.