Abstract: This paper begins briefly with a theoretical analysis of the propagation of seismie waves in fractured media, using a two-phase model of the media. Based on this preliminary theoretical result, and simple computational examples of wave propagation, it could be shown that possible changes in the form and amplitude of seismic waves in fractured two-phase media may be observed.Using observational data of explosions at the Maanshan Iron Mine and some small earthquakes occurred in Tangshan, Songpan, Mabian, Kangding and other seismic active regions, practical methods for studying anomalous amplitude or amplitude ratio before large earthquakes have been proposed. It is believed that the media surrounding their hypocenters were fractured or in two-phase.Some basic characteristics of amplitude anomaly, especially amplitude ratio anomaly appeared before those major earthquakes have been studied in detail.Finally several factors affecting the detection and analysis of amplitude anomalies are briefly discussed. Results and pre liminary conclusions thus obtained might be helpful for studies of earthquake prediction.