The ultra-high-strength stainless steel with high strength and toughness, high inherent fatigue strength and good corrosion resistance can be mainly used in some load- bearing components, such as landing gear, gas turbine engine shaft, and wheel bolts that working under corrosion environment. An environmental friendly ultra high strength stainless steel S280 for a new type of aircraft landing gear was developed in order to avoid the toxic effect of protective coating processes. The new ultra high strength stainless steel S280 has mechanical properties equal to, or better than conventional ultrahigh strength steels with the added benefit of modest corrosion resistance. By heat treatment process experiments and mechanical tests, the appropriate heat treatment process of S280 steel (Heating to 1080 ℃，holding at heat for one hour, and quenching in oil, cooling to -73℃, holding at this temperature for two hours, and warming in air to room temperature and then heating to 540℃, holding at heat for four hours, and cooling in air) was found in this paper. After heat treated under the appropriate heat treatment process,S280 steel can get good comprehensive performance, with the strength of 1900MPa, and KIC of 90MPa?m1/2. The new ultrahigh strength stainless steel S280, which apply low-carbon martensitic transformation hardening and aging strengthening superimposed, has high performance and can replace the existing Aer-Met100,300M steel material used for aircraft landing gear. The microstructures of new ultrahigh strength stainless steel S280 after thermal ageing at 540℃ and 600℃ was investigated by optical microscopy, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM).The results show that the S280 steel aged at 540℃ has desirable microstructures, which is fine lath martensite matrix with high-density dislocation and finely dispersed precipitate strengthening phase, and film-like reversed austenite precipitated from the boundary of martensite. The strengthening phases of the steel include Fe2Mo[1], Fe3C and Cr2C. There is no related report about that Cr2C precipitated from ultrahigh strength steel as a strengthening phase. In this paper, it was firstly discovered. And the crystallographic orientation relationship between Cr2C and matrix was measured: (1 -1 0 )M // ( -1 2 -1 )Cr2C, [ 1 1 1 ]M // [ 1 1 1 ]Cr2C The steel aged at the different temperature of 600℃ and 540℃ have the same precipitated phases Cr2C and Fe2Mo. The precipitated phases are relatively bigger than that of sample aged at 540℃.The strengthening phases of S280 steel have a high content of Cr and Co analyzed by energy spectrum analysis (EDS).