The phase diagram of the Pt-Sb system was investigated. Mixtures of Pt and Sb of 5 nine purity, which were vacuum sealed in silica capsules, were homogenized at high temperatures and quenched to prepare Pt-Sb alloys. Phases in the alloys were identified by optical microscopy, X-ray diffraction, and electron microprobe analysis. The existence of compounds, Pt34Sb7, Pt3Sb, Pt3Sb2, PtSb and PtSb2 was confirmed. A new phase, Pt34Sb7, was first found and had a composition of 82.7, 83.9 at.% Pt with a tetragonal Structure of the lattice parameter, a=3.948(3), c=16.85(1)A. PtSb is stoichiometric up to 800℃ and becomes Pt deficient as much as 1 at.% at 1000℃ Solid solubility limits of Sb in Pt were determined to be 7.5, 10.0, and 6.1 at.% at 1000, 800 and 600℃, respectively. The liquidus curve between the PtSb2 and Sb phases was revised.