A　new　molecular　phase　transition　material,　[PhCH2NH(CH3)(2)](2)C2O4　center　dot　H2C2O4,　which　undergoes　a　reversible　phase　transition　at　151.6　K,　has　been　successfully　synthesized.　Differential　scanning　calorimetry　(DSC),　specific　heat　capacity,　and　dielectric　measurements　confirm　its　reversible　phase　transition　with　a　large　thermal　hysteresis　of　15.1　K,　demonstrating　that　the　phase　transition　is　typical　first　order.　Variable-temperature　single-crystal　X-ray　diffraction　analyses　reveal　that　the　order-disorder　transformations　of　carboxy　oxygen　atoms　induce　the　structural　phase　transition.　A　slight　reorientation　of　the　oxalic　acid　unit　is　discovered　to　accompany　the　ordering　of　carboxy　oxygen　atoms　at　low　temperature.　The　DSC　measurement　result　of　the　deuterated　analog　is　different　to　that　of　1,　indicating　that　proton　dynamic　motions　in　hydrogen　bonds　also　contribute　to　the　phase　transition.