Alloy 709 austenitic stainless steel is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of deformation mechanisms on Alloy 709 and of tensile response of aged Alloy 709 is lacking. In this study, thus, the tensile behaviour of as-received and aged Alloy 709, their deformation microstructures and failure mechanisms, have been investigated at room temperature (RT), 550, 650 and 750 °C. Aging brought about the formation of particles at grain boundaries and interior of grain, thus leading to enhancement of yield strength but reduction in ductility. The ultimate strength of both materials is strongly temperature dependent, which clearly decreases with temperature. It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallisation at different temperatures. Display omitted •DSA effect was observed at 550°C in both as-received and aged materials.•Yield strength of the as-received alloy reduces with increasing temperature but no clear reduction in range of 550-750°C.•Aging increases the yield strength but reduces the ultimate tensile strength and ductility.