Abstract: On the basis of observational data of the vcertical geomagnetic component, this paper discusses the geomagnetic effect of the Tangshan earthquake of magnitude 7.8. The results of analySis indicate that before the Tangshan earthquake the vertical component had had distinct anomalous changes in the areas of Beijing, Tianjin and Tangshan. From September 1975, especially fiom December of this year, the vertical component in these areas had been decreasinggthe places where the changes were remarkable were located at Tangshan and Changli. Both ploces are just near the epicenter of Tangshan earthquake. From september 1975 to July or August 1976, the amount of decrease in Tangshan is about 7-10Y, while in Changli is about 16γ. Actually the geomagnetic anomaly in Tangshan began from October 1973. From October 1973 to May 1974 the vertical component had increased, the amount of increase is about 15γ.From May 1974 to August 1976 the vertical component had decreased, the amount of decrease is about 18-21γ. In this decreasing process, from February to December in 1975, the vertical component was temporally in a stable state. Peobably this state was affeced by the Haicheng earthquake of magnityde 7.3, 1975.