The mechanism for the formation of Mesoproterozoic mafic dike swarms in the central part of the North China Craton has been studied according to an analysis of the shapes and distribution characteristics of these dike swarms. The authors hold that they were mainly formed by emplacement of mafic magma into the earlier extensional or shear-extensional fractures. Most dike swarms in the northern Shanxi and Wutai-Taihang areas are shear-extensional, while most dike swarrns in the Luliang and Zhongtiao areas are extensional. According to the distribution, emplacement, mechanism and relation with the Yanliao - Zhongtiao aulacogen system, the Mesoproterozoic tectonic stress field in the central part of the North China Craton was extensional and shear-extensional, in which the principal compress stress probably stemed from the extension of the Yanliao-Zhongtiao aulacogen system. The existence of the undeformed and unmetamorphed Precambrian dike swarms shows that the Precambrian North China Craton had been a rigid plate.