In this paper,first,the stacking fault energy of the retained austenite in medium manganese Q&P steels respectively with a Mn content of 5% and 7% were calculated by using the thermodynamic model of stacking fault energy,and the effect of carbon partition during the heat treatment on the stacking fault energy was analyzed.Then,the deformation mechanism of the steel under tensile strain was investigated by means of XRD,SEM and TEM,and the relationship between the stacking fault energy and the deformation mechanism was explored.The results show that ( 1) carbon partition should be considered during the calculation of the stacking fault energy of medium manganese Q&P steels; ( 2) the stacking fault energy of 7Mn-1. 5Si-0. 2C sample is 4. 55mJ /m²,and that of 5Mn-1. 5Si-0. 2C sample is 17. 8 mJ /m² ; ( 3) there is a linear relationship between the natural logarithm of retained austenite content and the deformation; and ( 4) TRIP effect is the main deformation mechanism of medium manganese Q&P steels,which is also verified by the calculated results of the stacking fault energy.