Abstract: Sodium activity coefficient in liquid sodium amalgam has been determined by measuring the emf of the solid electrolyte cell Na|β-Al₂O₃|Na_xHg_1-x, for x=0.00001 to 0.055 at temperatures from 303.15 K to 383.15 K. The temperature dependence of the emf of the cells with various x values shows several small breaks located at ca. 318.15K, 333. 15K, 353.15 K and 370.85 K respectively. The discrepancies of the emf-temperature coefficients in different temperature-intervals decrease gradually with decreasing x values. Experimental parameter eguations determined for (1) emf of the cells (E)， (2) sodium activity coefficients in amalgams (γ_Na)， and (3) standard emf (E⁰) of the cell consisting of pure sodium and a hypothetical sodium amalgam with state extrapolated from a reference state at infinite dilution are as follows.