A new one-dimensional iodo-bridged mixed-valence diplatinum(11,111) complex (MMX chain compound) with dithiobutanoate (n-PrCS2-) as a ligand, Pt-2(n-PrCS2)(4)I, was found to undergo a structural phase-transition at 358-359 K by the differential scanning calorimetry (DSC) measurement and X-ray crystal structure analyses, In X-ray diffraction photographs measured using synchrotron radiation of the SPring-8 facility, the weak but distinct Bragg spots corresponding to the periodicity of 2-fold repetition length of a -Pt-Pt-I- unit were observed at 300 K and changed to diffuse scattering at 350 K. The electric conductivity deviates from the typical semiconducting behavior in the temperature region above 330 K.