A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presented on the basis of thermodynamic calculations.This is an integrated model,which includes one for calculating the retained austenite(RA)fraction and the other for volume expansion during the austenite-to-martensite transformation,because both of them are key parameters for calculating the density of steel at ambient temperature.The existing empirical equations for calculating M s temperature and lattice constants of both martensite and austenite have been all reassessed by the XRD measurements on the microstructures of seven hot-rolled medium-Mn steels.Finally,the densities of seven steels were calculated merely from compositions and compared with the measured ones.The difference between them is no more than 1%,suggesting that the presented model should be of good value in designing the low-density steels. A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presented on the basis of thermodynamic calculations. This is an integrated model, which includes one for calculating the retained austenite (RA) fraction and the other for volume expansion during the austenite-to-martensite transformation, because both of them are key parameters for calculating the density of steel at ambient temperature. The existing empirical equations for calculating M s temperature and lattice constants of both martensite and austenite have been all resessed the XRD measurements on the microstructures of seven hot-rolled medium-Mn steels. Finally, the densities of seven steels were calculated merely from compositions and compared with the measured ones. The difference between them is no more than 1%, suggesting that the presented model should be of good value in designing the low-density steels.