The relaxation test of molten slag (CaO-SiO 2-Al 2O 3) was carried out to evaluate its mechanical properties. Slag were compressed at the constant compression rate till the particular displacement distance, then the compression process was stopped and the stress relaxation of molten slag was measured with time. It was found that the relaxation stress strongly depends on the initial compression rates. All of the stress relaxation curves show the same variation with time just as that with the compression of 30 mm/min. This observed relaxation behavior was quite different from that of Newtonian fluid since it can keep the internal stresses.