Abstract: The tectonic stress field in the lithosphere is divided into three layers in the Qinghai-Tibet plateau, i. e. the lower lithosphere, the seism ogenic layer and the shallow curst. In addi-tion to that stress directions in the uppertwolayers can beestimated respectively using the focal mechanismsolution methodor in-situmeasurement methods, the method bisecting conjugateang les of plastic-flow belts is suggested toestimate those in the lower lithosphere in which tw of amilies of plastic-flow belts intersect eachother forming plastic-flow network. The com parison between the tectonic stress fields in the lower lithosphere and the seismogenic layer indicates that the stress directions in the upper layer are identical with and controlled by those in the low-er layer in general tendency because the long-rang e transmission of plate-boundary driving force carried out mainly by the netlike flowin the lower lithosphere, while the considerable differ-ences between the upper and lower layers exist in some local areas nearthe Himalay and riving boundary, where a part of the driving force is transmitted directly along the upper layer.

Key words: Qinghai-Tibet plateau, Lithosphere, Plastic-flow network, Seismogenic layer, Tectonic stress field