There are four types of Mesozoic igneous rock assemblages in western Shandong. Five 40Ar/39Ar plateau ages of representative rocks from three assemblages were determined. The four assemblages are as follows: a—quartz monzodiorite-monzonite porphyry and syenite porphyry assemblage: the representative Tongshi complex was emplaced in Paleozoic strata and unconformably covered with Middle Jurassic Wennan Formation; the plateau age of hornblende from quartz monzodiorite porphyry is 189.8±0.2 Ma, whereas that of hornblende from monzonite porphyry is 188.4±1.6Ma. b-pyroxenite-monzodiorite-syenite assemblage: the representative Shagou intrusive body was emplaced in Archean granite, and the plateau age of biotite from pyroxenite is 115.1±1.1Ma. c—olivine noritic gabbro-two pyroxene dioritequartz monzonite assemblage: the representative rock body in Zhouping area was emplaced in volcanic rocks of Cretaceous Qing-shan Group; the plateau age of biotite from olivine noritic gabbro is 112.5±1.3Ma,while that of biotite from quartz monzonite is 115.1±0.3Ma. In early years, 67 individual mineral and whole rock K-Ar ages for this assemblage as well as one Rb-Sr isochron age and one Sm-Nd isochron age were determined. Statistics show that the modal age values are in the range of 110-132Ma, which are comparable with our40Ar/39Ar ages. d—diorite porphyry-quartz monzodiorite porphyry-granite porphyry assemblage, exemplified by the rock bodies in Tongjing area; no individual mineral samples suitable for40Ar/39Ar age determination have been collected in this area, and the age values avaibable are concentrated in the range of 110-126 Ma. There existed two periods of magmatic activities in western Shandong: the Early Jurassic igneous activity seemed not so strong and was merely confined to the southern part of western Shandong, forming K-Ca-high alkaline—K-high alkaline rock series; the Early Cretaceous igneous activity was ubiquitous in the whole area: the southern part of western Shandong shows features of alkali-high magma, whereas in the northern part, the early rocks are of calc-alkali ones, but the late rocks have higher potassic content. The igneous rock assemblage near Yishu faulted zone is characterized by well-developed granitic rooks.