Abstract: On the basis of aeromagnetic data the Curie depth over the Bohai Sea water area was calculated by using the method of spectral analysis of residual anomalies described by Prof. B. K. Bhattacharyya. The results obtained indicate that the crustal temperature under the sea is rather higher and in some places the Curie point is located only at a depth of 12km.The idea that the Bohai Sea area can be viewed as a high point for upwelling of materials in upper mantle beneath the North China region was formulated by many researchers in geoscience according to the geological and geophysical data from the Bohai Sea and the surrounding areas. And later it was confirmed to be correct by the calculated results mentioned above. Therefore, the high heat values that appeared in the Bohai depression and the surroundings are not occasional but have a deep-seated background. Thus the upper mantle u’pwarping from the North China block fault, leading to thinning out of crust and subsiding and squeezing of heated materials into crust along the faults, all promoted the thermal action, in which the Tancheng-Lujiang fault zone would play a specially important role. The upwelling mantle materials through this fault zone resulted in a gradual increase (from west to east) of the temperature field in the North China region and reached its maxium over the Bohai Sea area.This paper indicates that the earthquakes having occurred in the North China region including the Bohai Sea area cannot be fairly explained simply by the stress field formed in collision of some lithospheric plates around the Chinese mainland. The horizontal gradient of temperature field generated during the upward flow of deep-seated heat materials due to the non-homogeneity of earth crust is closely related to the seismicity in the North China region.