A novel stable tetrasila-1,3-diene 3 was synthesized by the reduction with sodium of the corresponding 1, 2,2-tri bromod isi lane. X-ray analysis shows that the tetrasila-1,3-diene skeleton of 3 is highly twisted but adopts an anticlinal conformation in contrast to the previously reported tetrasila-1,3-dienes showing a synclinal conformation. Upon heating at 80 C or irradiation using filtered light (lambda > 390 nm) of 3 in benzene, clean dissociation of one of the Si=Si double bonds of 3 occurred to give a novel cyclotrisilene 4 together with silylene 2 suggesting the initial Si=Si double bond dissociation of 3 to the corresponding disilenylsilylene 8 and silylene 2; cyclotrisilene 4 would be formed by the intramolecular silylene insertion into the Si=Si bond of S.