Inner Mongolia has vast territory and, according to the theory of plate tectonics, strides across Siberian, North China, Kazakhstan and Tarim plates. Its tectonic environment is extremely complex in that it has experienced multi-period structural movements in geological history. For years, the boundary between North China and Siberian plates has been a discussion hotspot among geologists. Based on an analysis of the geophysical gravity and magnetic field characteristics of the suture zone, the authors hold that Xar Moron River fault zone should be the ultimate suture zone between North China and Siberia plates. The results obtained by the authors provide geophysical evidence for the determination of the ultimate suture zone between the two large plates.