Elevating steam parameters is the key to enhance the efficiency of fossil power plants, reducing fuel consumption and noxious emission. Therefore, a lot of new creep resistant martensitic stainless steels have been developed, among which FB2 steel (a new 9%Cr martensitic stainless steel) is the most promising candidate for manufacturing steam turbine rotors operated at temperature range from 600 ℃ to 650 ℃. In the present work, the evolution behavior of Laves phase in the as received FB2 steel was studied by thermal simulation technique. Firstly, some sparse micron-sized particles of Laves phase were observed in as received FB2 steel by SEM. It was concluded that the large Laves phase particles formed in casting due to dendritic segregation. Then constitutional liquation resulting from eutectic reaction between Laves phase and γ-Fe in the heating process of welding thermal simulation was found, suggesting a liquation crack tendency in heat affected zone of FB2 steel. In the specimens experiencing thermal simulation, some eutectic microstructures were observed by SEM. Based on the results of EDS analysis and SAED, the two eutectic constituents were identified as χ phase and γ-Fe. At last, the reason for the difference in morphology of eutectic microstructures between specimens experiencing thermal simulation with different peak temperatures was analyzed.