A significant NNW striking bimodal dike swarm was discovered in Nankou area of Beijing, which is composed of basic shoshonite and acidic granitic rocks with K-Ar isotopic ages of 119-114Ma. The geological and petrological features of the dike swarm are investigated in this work. The acidic and basic dikes in the swarm have very similar REE patterns. And both of them do not have obvious Eu abnormality, indicating that no correlation of crystal fractionation exists between the magmas forming the acidic and basic dikes. Thus it is suggested that they came from different magma sources with genetic relationship and such a dike swarm may be referred to as bimodal. Certain degree of hybridism on the borders shows that both kinds of magma intruded almost at the same time when they were both in liquid state but still independent from each other in the whole. The attitude and geochemical features of the deep-source shallow-intrusion dike swarm reflect a Late-Mesozoic extensional tectonic background for the magma intrusion in the concerned area, which sheds some light on completely understanding the Yanshanian Movement in North China.