Abstract: We review the recent development of uranium-thorium disequilibrium technique and its application to young volcanic rocks from eastern China. All basalts from eastern China show significant 230Th excesses, indicating deep and slow mantle melting in the garnet stability field. Recent addition of subduction-related fluids from the Pacific plate is not supported by U-Th disequilibrium in young volcanic rocks. Measurement of U-Th disequilibrium in zircons in young evolved volcanic rocks provides insights into magma residence times before eruptions. The zircons from Tianchi of Changbaishan are only 9000 years (9ka) old, predating the millennium eruption by only 8000 years. In comparison, the Maanshan zircons show two age populations in 55ka and 91ka. The short residence time of the Tianchi volcano suggests that Tianchi volcano is highly dangerous. The rapid development of the great Millennium eruption is consistent with the trigger mechanism of the great Millennium eruption by injection of basaltic magma into the comenditic magma.