Differential scanning calorimeter (DSC) experiments were performed on a solid-solution strengthening Ni-base superalloy 625, considering the effects of the powder particle size (<37μm, 45-53μm, 75-105μm, 105-150μm, 150-355μm) and microstructure on the phase transformation temperature. The alloy powders were characterized by FESEM, EPMA and synchrotron XRD. The results indicate that the dendritic structure is evident in powders with different particle size and the dendritic arm spacing is in the 2-10μm range. Elements Ni and Cr are rich in dendritic core whereas the Mo and Nb tend to distribute in the interdendritic region. Only the matrix γ phase exists in the PM625 powders with different particle size range. The PM625 powders with weak segregation tendency exhibit a sharp inflection point in DSC heating curves in the region near solidus temperature, there is only a 2-5°C gap between the incipient melting temperature of the alloy (deviation from the baseline inflection point) and the nominal solidus temperature (tangent-onset intersection) for different particle size. However, the gaps between the norminal solidus and the end of the solidification temperatures are relative large, which is in 53-65℃ range, in DSC cooling curves, because the low segregation characteristic of original powders has been removed during the full remelting and re-solidified process. The differences in solidus, liquidus and incipient melting temperatures in DSC heating curves are maximum 3℃, 2℃ and 2℃ among different particle size powders, whereas they are 6℃ and 2℃ for the solidus and liquidus temperatures of the alloys in the cooling curves. Therefore, the particle size has minor effect on phase transformation temperature of solid-solution strengthening PM625 alloy powder.