Tensile tests were performed on specimens in quenched and tempered (Q + T) and thermally aged (TA) conditions over a wide temperature range (300-873 K) to assess the influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel. Prior thermal ageing at 793 and 873 K for durations ranging from 10 to 5000 h did not cause a significant change in room temperature tensile properties. However, a marginal decrease in yield strength and reduction in area were observed for specimens aged for longer durations at 793 and 873 K. Prior thermal ageing at 793 K for 5000 h and at 873 K for 1000 and 5000 h produced significant reduction in strength values at intermediate temperatures (523-723 K) compared to that observed at high temperatures. At intermediate temperatures, the alloy in all heat treatment conditions exhibited serrated flow, a manifestation of dynamic strain ageing (DSA). The significant loss of strength in thermally aged conditions at intermediate temperatures has been attributed to reduced propensity to DSA. The elongation to fracture values at temperatures in the range 300-873 K were affected little by prior thermal ageing, whereas the reduction in area exhibited a decrease in the value with increasing thermal ageing. The fracture mode remained transgranular ductile at all test conditions investigated in the present study. However, specimens aged for longer durations exhibited chisel fracture at room and intermediate temperatures due to split in the martensite lath boundaries. The influence of thermal ageing on room temperature tensile properties of the forging remained similar to that reported for thin section 9Cr-1Mo steel.