Fundamental properties of ammonia borane (AB) aqueous solution were experimentally investigated in this study. The enthalpy change of dissolution ∆dissolH° for AB aqueous solution was 17.1 ± 0.9 kJ/mol, which was evaluated by heat of dissolution at each concentration. The freezing points and solubility curve were obtained by the concentration and density of AB aqueous solution at each concentration and temperature. The eutectic and transformation temperatures of AB aqueous solution were obtained by thermal analysis which were cooling/heating curves and heating profiles by differential scanning calorimetry (DSC). The stabilities (shelf-life) of 0.1 mol/L AB aqueous solution at room temperature and 8 mol/L solution at 30 and 40 °C were evaluated. It was suggested that the AB aqueous solution should be stored with non-exposure to air at room temperature by a year but less than 40 °C. From the experimental data, a phase diagram was drawn by the freezing points, solubility curve, and thermal analysis. Respectively, the estimated eutectic concentration and temperature from this phase diagram was found to be 8.93 wt.% (2.75 mol/L) and -5 °C. Based on these results, it is suggested that AB aqueous solution should be handled at above -5 °C and below 40 °C.