The orthorhombic-tetragonal structural phase transition behaviors of Pr2Ni_O4+δ and Nd2NiO_4+δ were investigated by DSC and TG-DTA under controlled oxygen partial pressure, P(O₂), and their difference was discussed. Endothermic peak corresponding to the structural phase transition was observed in DSC curves. The transition temperature, T_p, decreases with decreasing P(O₂) both in Pr₂NiO_4+δ and Nd₂NiO_4+δ. A discrete weight variation originating from the variation of oxygen content, Δδ, was observed during the phase transition both in Pr₂NiO_4+δ and Nd₂NiO_4+δ. The Δδ of Pr₂NiO_4+δ was larger than that of Nd₂NiO_4+δ, and the Δδ of Pr₂NiO_4+δ increased with decreasing P(O₂), while that of Nd₂NiO_4+δ was independent on P(O₂), suggesting that not only the valence of Ni but also that of Pr changed during the phase transition in Pr₂NiO_4+δ. From the Ellingham diagram prepared using Tp measured by DSC under various P(O₂), ΔH° and ΔS° at the phase transition were evaluated. It was revealed that ΔH° of Nd₂NiO_4+δ is larger than that of Pr₂NiO_4+δ, whereas ΔS° shows little difference, showing agreement with higher Tp of Nd₂NiO_4+δ that that of Pr₂NiO_4+δ.