Cenozoic ultrabasic-basic dyke swarms were newly discovered in northern Mitizi area in northwest Tibetan Plateau. The dyke swarms intruded into the Changchengian Sailajiazitage Group. The nearly E-W-trending dyke swarms are located on the northern margin of Kogan Fault. The rocks are mainly composed of spessartite and kersantite, which have features of AL_Na and CAL_Na respectively, as shown by geochemical analyses. The LA-ICP-MS zircon U-Pb age of spessartite yielded an age of 38.24±0.54 Ma, suggesting that these dyke swarms were formed in Paleogene. The dyke swarms have low REE (64.28×10^-6~253.41×10^-6), and the anomalies of Eu (δEu=1.04~1.21) and Ce (δCe=0.99~1.03) are not obvious. REE distribution patterns are of the right-oblique type (LREE/HREE=4.15~6.45). All samples are characterized by the relative enrichment of LILE and depletion of HFSE. The dyke swarms are products of low degree partial melting of lithospheric mantle. The magmatic activity lasted for a short time and the dyke swarms had quick cooling rate after intruding into the surrounding rocks. The tectonic setting of the dyke swarms was strike-slip fault, and the magmatism occurred along the large scale strike-slip faults.