The purpose of this thesis is, through a series of arguments about a federal system by Carsun Chang (Zhang Junmai, 1887–1969) and Zhang Dongsun (1886–1973) who were two of core members of the Research Clique (Yanjiuxi), to analyze a formation of its thought as a group. In second half of 1910's, Zhang Dongsun often contended federal system both in theory and practice. Zhang insisted local autonomy is the only way to protect autocracy or rule by warlord. But Carsun Chang objected to Zhang Dongsun's opinion, asserted that a local government is merely an administrative distinct which should be persistently guided by the central government. However, Carsun Chang gradually changed the evaluation to federal system, via study in Germany from 1913 and inspection around Europe from 1918. Meanwhile, Zhang Dongsun continued to insist on federal system. Then, they published a series of letters called ”China's future: Germany or Russia?” in June 1920, where their opinions about federal system were almost coincided in theory and practice. This is because of the difference of their political standpoint, overseas experience, and influence from Liang Qichao. Author concluded that, June 1920 is a date when their opinion about plans of China agreed, and simultaneously, when their opinion about federal system almost coincided is certainly a phase of formation of the Research Clique's thought as a group, and this date, at the same time, ironically, is after their political career was broken off.