Mechanical performance of reduced activation ferritic/martensitic (RAFM) steels for fission and fusion applications at high temperature (i.e. above the current limit of 550 .C) requires further improvement. In this contribution, we present the work aimed to improve the high temperature strength and creep resistance of RAFM steels. Two RAFM steel grades were developed based on thermodynamic modelling targeted to optimize the carbonitride precipitation distribution. A high content of carbonitride formers was considered to ensure high fraction of finely dispersed MX-type precipitates. Following the chemical tuning guided by thermodynamic modelling, the screening of different tempering conditions was performed in a high-throughput mode to find the optimal heat treatment for each model alloy. The model alloys were investigated in detail to assess the microstructure, tensile and impact properties. The results are used to validate the improvement of the new grades compared to the reference material EUROFER97. Furthermore, the results are compared with recent developments in high temperature RAFM steels obtained by other research groups.