Abstract: Grain ultra-refinement of cyclic phase transformation the microstructure and mechanical properties of 00Cr13Ni7Co5Mo4Ti maraging stainless steel after being refined were studied. The results show that cyclic transformation treatment at differnet temperature could refine grain from 180 μm to 6 μm for 00Cr13Ni7Co5Mo4Ti maraging stainless steel, gave refined and equiaxed grains . The large martensite laths were partitioned into several parts by some smaller clusters of martensite, which refined martensite microstructure. The steel after cyclic phase transformation at differnet temperature showed an increase of 170 MPa in yielding strength, 100 MPa in tensile strength, 18% in elongation percentage, and 13% in contraction area, respectively than that treated by traditional high temperature solid solution and subsequent aging.