Zero thermal expansion (ZTE) materials are rare, but have intriguing properties which can be used in precise optical devices to prevent thermal fluctuation. Here, a new cube ZTE compound K₆Cd₃(C₃N₃O₃)₄, composed of conjugated [C₃N₃] rigid six-membered-rings and [CdN₄] tetrahedra, has been reported for the first time. Its thermal expansion coefficient is as small as 0.06 MK⁻¹, which is one-order of magnitude lower than that of the commercially available Invar alloy. In addition, the ZTE mechanism was elucidated using first-principles calculations.