Titanium-stabilized 15Cr-15Ni austenitic stainless steel in 20% cold-worked condition has been considered as a promising candidate high temperature structural material for core components in liquid metal-cooled fast reactors (LMFR). Microstructure and high temperature mechanical properties of as cold-worked and different aging treatments alloys were investigated by using metallographic microscope, scanning and transmission electron microscope, and tensile testing machine. The results demonstrate that the recovery took place when the alloys were aged at 550℃ for 336h. The emergence abundant of twins was attributed to dislocations movement. When aging temperature increasing to 650℃ and 750℃, the amount of twins reduced because of alloy′s further recover. Sigma phase precipitated from austenitic matrix along grain boundaries when alloys were aged at 650-750℃ for 336h. The amount and size of sigma phase increased with the aging temperature and time increasing. Because of the grain boundary strengthening, alloys aged at 550℃ had slightly higher yield strength and ultimate tensile strength than those of as-cold worked alloy. The high temperature yield strength and ultimate tensile strength decreased significantly, while plasticity increased observably when aging temperature increasing to 650℃ and 750℃.