The effect of NiAl-type nanoparticles on the austenite stability was investigated during quenching-partitioning-tempering (QPT) processes in a cold rolled medium-manganese steel. A good combination of ductility (total elongation: 37.9%) and strength (yield strength: 995 MPa/ultimate tensile strength: 1260 MPa) is obtained after the first step of partitioning treatment at 630°C/1 h (P630) due to appropriate austenite stability and multiple strengthening mechanisms. Moreover, the yield strength and ductility increase further after the second step of tempering treatment at 500°C/2 h by about 113 MPa and 8.5% respectively. It was found that high density of intragranular NiAl-type nanoparticles precipitated during tempering improve the ductility from two aspects. NiAl-type nanoparticles could provide a harder and work-hardenable martensite matrix, which is beneficial for the stability and sustainability of the retained austenite during tensile deformations.