The effect of temperature on the lattice constants of martensite and ferrite are studied by the synchrotron radiation technique. The results show that the lattice constants of martensite and ferrite increase gradually with the increase of temperature, but diffraction peaks of martensite show a peak splitting phenomenon, while the diffraction peaks of ferrite do not. From the fitting of the 110 and 200 diffraction peaks of martensite, it is found that the lattice constants a and c of martensite gradually increase with the temperature rising, but the increasing rate of lattice constant a is faster than that of c, that is, the squareness of martensite gradually decreases. When the temperature increases to 500 ℃, the squareness (c/a) of martensite is equal to 1. The variation of lattice constant of ferrite with temperature is basically the same as that of lattice constant a of martensite, but different from that of lattice constant c of martensite, showing the nature influence of temperature on the lattice constants of martensite. Besides, based on the analysis of synchrotron radiation data, the quantitative equations of lattice constants of martensite and ferrite varying with temperature are established.