Y-junction carbon nanotubes have been grown by hot-filament chemical vapor deposition for which a gas mixture of acetone and hydrogen was fed and in situ evaporated copper was supplied. Transmission electron microscopy images reveal that? two of three branching angles around the Y-junction are obtuse (>90 degrees) while the other is sharp (<90). The sharp branching angles ranging between 50 degrees and 80 degrees are nearly twice the bending angles of simple bend junctions. This indicates that the Y-junction can be presented in structure as a combination of two bend junctions. An atomic configuration involving six heptagons on the three saddle surfaces is proposed to understand the topological structure of the observed Y-junction carbon nanotubes.