Abstract: Tectonic forces from the relative movements between plates are transmitted into the continental crust, and then they create the earthquake generating stress field there. The space - time distribution of the seismic activity including the small earthquakes in a region reflects the variation of the stress field in the region.According to this idea, the characteristics of the stress fields in the various regions of East Asia have been analyzed in detail in this paper based on a lot of solutions of focal mechanisms and data of seismic activity during the last 500 years. The results indicate that the tectonic forces from the subduction of the Pacific Ocean plate underneath the Eurasian plate control the stress field in the region from North China to the northern part of the North- South Seismic Belt.The variation of the regional stress field shown by the variation of seismic activity in some regions of Japan has also been discussed based on characteristics of variation of the seismicity of small earthquakes.Synchronous variations of seismicity in the past 100 years or so in West China and in the boundary region between the Indo - Australian and Eurasian plates implicate that there is the transmission of tectonic forces into West China through the collision between the Indo - Australian and Eurasian plates. The active seismic activity in the boundary region between the Indo - Australian and the Eurasian plates and in West China is continuing consistently.