Instrumented indentation technique has been utilized to assess the flow properties in X20CrMoV12.1 steel, widely used in power generation facilities, at various heat treatment stages simulating thermal aging during the service. The steel samples were heat treated at 600 and 650 A degrees C for 1-2,000 h and flow properties were evaluated at various heating temperature and times by instrumented indentation tests using spherical indenter. Microstructure evolution after various heat treatments has been investigated to correlate the degradation of flow properties with the microstructure evolution due to thermal aging. We show that the degradation in flow properties in the steel sample can be described by Larson-Miller parameter analysis.