The long range order parameter of the β\u27AuZn alloy of the L2_0-type structure has been measured by X-ray diffraction at temperatures ranging from 200℃ to 650℃, the latter temperature being 75℃ below the melting point T_m. Relative intensities of several h00 type reflections from single crysta]s have been measured using a counter diffractometer equipped with a high temperature attachment. It has been observed that the order parameter gradually decreases with rising temperature but appears to keep an appreciably high value at T_m. An abnormal increase in the lattice spacing has also been observed at about 550℃, which corresponds to the onset of an appreciable disordering detected by the intensity measurement. It is concluded that β\u27AuZn is a special type of ordered alloy and it would undergo a transition into the disordered alloy state, if it were not melted. The root-mean-square displacement of atoms due to thermal vibration has been measured as a function of temperature. At room temperature the displacement is 0.125A, 4.6% of the nearest neighbor distance, and increases to 0.255A at 650℃
