Abstract: The characteristics of anomalies for all the data of earth resistivity during the three earthquake groups occurred in North China (Tangshan group occurred in 1976, Datong group occurred in 1989, Zhangbei group occurred in 1998) are analyzed. The results show: (1) the duration of resistivity anomaly for earthquake group is relatively longer, and the anomaly curves usually keep decreasing continuously or staying at the low value after the main shock, which makes the tendency background very clear during the whole course. (2)The number of station that has anomaly is large and its distribution is wide before main shock. However, anomalies of aftershocks concentrate in focal region. (3) Anomalies recover very quickly after the earthquake group. The relationship between the development of anomaly and the mechanism of earthquake preparation is discussed at the end.