Biotite and plagioclase samples from the quartz monzodiorite of Xinwuli granitoids yield ⁴⁰Ar-³⁹Ar plateau ages of (152.1±1.7) and (135±1) Ma. Samples from the granodiorite give ⁴⁰Ar-³⁹Ar plateau ages of (143.8±0.9) Ma and (140.4±0.9) Ma. The similar ages suggest an important stage of Yanshanian magmatic activity in the Middle and Lower Yangtze regions. The ⁴⁰Ar-³⁹Ar age of biotite from quartz monzodiorite is older than the previous zircon U-Pb isotopic ages, which is probably due to excess argon. Together with the age data of Xinwuli granitoids, the authors obtain the cooling curves of the granodiorite and quartz monzodiorite.