A group of Mesozoic granitoid samples from southeastern China were systematically collected and measured, and 65 feldspar lead isotopic data were obtained. In combination with published data, the possiblity of existence of Cathaysia is discussed according to the principle that the lead isotopic composition of feldspar can reflect the chemistry and composition of basement rocks. The data indicates that the basement rocks of southeastern China can be divided into three blocks, i. e.:I. the Wuyi block (B4-2 lead isotope province), which is bounded by the Jiang-Shao fault zone on the north, by the Ganjiang-Heyuan fault on west and by the central and southern sections of the Zhenhe-Dapu fault on the east and whose 206Pb/204Pb, 207Pb/ 204Pb and 208Pb/204Pb are 18.226, 15.620 and 38.725 respectively ;II. the Fujian-Tai-wan block (B4-2 lead isotope province), covering an area to the east of the central and southern sections of the Zhenhe-Dapu fault and having 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 18.534,15.656 and 38.866 respectively,III. the Nanling-Hainan block (B3-1 lead isotope province), located west of the Ganjiarig-Heyuan fault, with three corresponding lead isotope ratios being 18.676, 15.688 and 38.924 -respectively. According to the relationships among the lead isotopic compositions of feldspar from, the above-mentioned three block, it is proposed that the fundamental factor causing the differences among them was the dynamic process for the U-Th-Pb system of source materials in particular tectonic and geochemical settings. Consequently "Cathaysia" never existed, and actually it was formed by amalgamation of sthree basement blocks with different natures and ages. The Fujian-Tatwan block is an accretion zone along the Wuyi block, while the Taiwan block was separated from the Fujian-Taiwan block in the Late Mesozoic.