Carbon honeycomb is a 3-dimensional carbon allotrope experimentally discovered recently but its lattice structure has not been well identified. In this paper we perform density-functional theory (DFT) calculations to examine the stability of carbon honeycombs with different configurations (chirality and sidewall width). We find that graphene nanoribbons with both zigzag edges and armchair edges can form stable carbon honeycombs if sp(3) carbon-carbon bonding is formed in the junction. We further study the mechanical properties and the thermal conductivity of carbon honeycombs with different chirality and the sidewall widths using both DFT calculations and molecular dynamics simulations. All these stable carbon honeycombs exhibit superior mechanical properties (large strength and ductility) and high thermal conductivity (larger than 100 W/m K) with a density as low as 0.5 g/cm(3). Light-weight carbon honeycombs could be promising functional materials for many engineering applications. (C) 2017 Elsevier Ltd. All rights reserved.