Modern martensitic 9-12% Cr steels are alloys with excellent mechanical properties even at elevated temperatures. The high temperature strength of these materials is inevitably related to their complex microstructure. Due to diffusional processes however, this microstructure changes during high temperature service, which leads to a decrease in strength. This work was aimed at modeling the 9-12% Cr steels with tailor-made microstructures for applications such as fossil fuel fired power plants, internal combustion engines etc. The investigations concentrated in the design and characterization of heat resistant steels for applications in high oxidizing atmospheres. A microstructure-property link is formulated with focus on the precipitate and solid solution hardening effect. For different heat treatments, the numerical results are compared with other samples. The numerical simulation showed excellent agreement in the case when all operative strengthening mechanisms are duly considered. The experimental results can be reproduced in a comprehensive and consistent manner by the numerical simulations using the software as MatCalc and ThermoCalc. From the result of the simulation, the observed phases after creep (M23C6 carbides, V-MX and Nb-MX particles and Laves phase) are in good agreement with the MatCalc and ThermoCalc calculations except for the Z-phase phase. The volume fraction of precipitated M23C6 carbides is directly related to the carbon content of the alloys.