We identify by first-principles calculations a new cubic carbon phase in I4132 (O 8) symmetry, named K 6 carbon, which has a six atom primitive cell comprising sp 3 hybridized C3 triangle rings. The structural stability is verified by phonon mode analysis. The calculated elastic constants show that the K 6 carbon is a high ductile material with a density even lower than graphite. Electronic band and density of states calculations reveal that it is a metallic carbon allotrope with a high electronic density of states of 0.10 states/eV per atom at the Fermi level. These results broaden our understanding of the structural and electronic properties of carbon allotropes.