The phase diagram of the rare-earth system SmCl3-PbCl2-MgCl2 has been investigated by DTA. The results of the ternary system are as follows : four liquidus surfaces corresponding to the primary crystallization of SmCl3, PbCl2, MgCl2 and PbSm3C11, respectively, five secondary crystallization lines, the ternary eutectic point E = 695 K (25.0 mol% SmCl3, 57.0 mol% PbCl2), and the ternary peritectic point P = 713 K (31.5 mol% SmCl3, 48.0 mol% PbCl2). The reactions between four equilibrium phases are:L-p+SmCl3 reversible arrow PbSm3Cl11+MgCl2L-E reversible arrow PbSm3Cl11+PbCl2+MgCl2.