The structures and reactions of 4-silatriafulvene were studied in detail by ab initio molecular orbital theory. At the equilibrium structure, 4-silatriafulvene shows severe bending about the Si=C double bond. A planar structure having two conjugated double bonds (an Si=C and a C=C double bond in the cyclopropenyl moiety) is the transition state for flopping between the two degenerate bent structures; the activation energy is within 1 kcal/mol at MP2/6-311++G** + ZPE. It is concluded that 4-silatriafulvene is balanced by two stabilizing factors, aromaticity of the cyclopropenyl moiety and Si=C double-bond formation. The remarkably low reactivity of a 4-silatriafulvene toward water that was found experimentally is reproduced theoretically by comparing the transition structures and activation energies with those of silaethene + water. Intramolecular isomerization from 4-silatriafulvene to silacyclobutadiene via cyclopropenylsilylene is even competitive with the water-addition reaction.