Hydrogenated silicon clusters (SinHm) were detected during the thermal decomposition of disilane in a flow reactor by using time-of-flight (TOF) mass spectrometry coupled with vacuum ultraviolet (VUV) photoionization at a temperature range of 680-860 K and total pressures of 20-40 Torr. SinHm clusters were photoionized by the VUV radiation at 10.2 eV (121.6 nm). The SinH2n+2 (n greater than or equal to 2), SinH2n (n greater than or equal to 2), SinH2n-2 (n greater than or equal to 5), and SinH2n-4 (n greater than or equal to 5) containing up to 10 silicon atoms were observed as gas-phase products during the disilane pyrolysis, while no ion signal due to Si-1 species such as Si, SiH, SiH2, and SiH3 was detected. Energetics for the disilane unimolecular decomposition has been calculated at the Gaussian-3 (G3)//B3LYP level of theory. On the basis of a kinetic model proposed by Swihart and Girshick [J. Phys. Chem. B 1999, 103, 64], the gas-phase kinetic simulation has been performed to analyze the formation mechanism of hydrogenated silicon clusters in the thermal decomposition of disilane. Temperature and time dependences of disilane loss and hydrogenated silicon cluster formation were compared with the results of kinetic simulation. The formation processes of Si2nH2n and SinH2n-2, which have linear or cyclic and polycyclic structures, were suggested to be the key steps for the large cluster growth.