A new phoshpate binder was developed for dental investments using the setting reaction between aluminium monophosphate, Al(H₂PO₄)₃ and aluminium oxide hydrate, Al₂O₃・nH₂O at an ambient condition.In this experiment, Al(H₂PO₄)₃ commercially available in both powder and liquid forms(50wt%)were used and Al₂O₃・nH₂O was synthesized by controlling the pH of a 10% aluminum sulfate salt solution.According to the X-ray diffraction method, they were amorphous at pH7.0 and were crystal at pH10.0.The powdered Al(H₂PO₄)₃ was mixed with Al₂O₃・nH₂O in the range of 30 and 70wt% an spatulated with water.The liquid Al₂O₃ was directly spatulated with Al₂O₃・nH₂O.In this study, the setting time of the set binders was measured for the evaluation of the reactivity of Al₂O₃・nH₂O.The reaction products were examined by the X-ray diffraction method and SEM.In the case of powdered Al(H₂PO₄)₃, the setting time became shorter with increasing amount of Al₂O₃・nH₂O in both forms.Furthermore, the amorphous Al₂O₃・nH₂O gave a faster setting time than the crystalline one.The mixture of Al₂O₃・nH₂O and powdered Al(H₂PO₄)₃ showed the highest compressive strength(90kgf/cm²)at the molar ratio Al₂O₃/P₂O₅=1(40wt% Al₂O₃・nH_₂O).The reaction products were mostly in amorphous phase except for the set binder containing crystalline Al₂O₃・nH₂O. In the case of liquid Al(H₂PO₄)₃, the setting time was similar to that for the powdered Al(H₂PO₄)₃. Their compressive strength increased with the increase in the molar ratio of Al₂O₃/P₂O₅ ratio.The reaction products were amorphous in all cases.The SEM graphs of the set binder containing amorphous Al₂O₃・nH₂O, revealed hole-like structure instead of dense structure observed in the set binder containing the crystalline Al₂O₃・nH₂O. The binders containing amorphous Al₂O₃・nH₂O(30〜70wt%)showed a setting time of 0.2〜6hours and the compressive strength of 90〜130kgf/cm² which is high enough for use as dental investments.