Magnetic properties of FeRh1-xPtx system with body-centered cubic and body-centered tetragonal (bct) structures were studied by means of X-ray diffractometry, magnetometry and Mossbauer spectroscopy, and the phase diagram in a temperature-composition plane was determined. With substituting Pt for Rh, the axial ratio c/a was increased and the bct phase was stabilized. With increasing temperature, the bct alloys with 0.72