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A lonsdaleite-like orthorhombic structure B2CO(oP8-B2CO)with strong sp3 covalent B‐C andB‐O bonds has been predicted theoretically by using the crystal structure prediction package CALYPSO.The mechanical,dynamical and thermodynamic stabilities of oP8-B2CO are verified by independent elastic constants,phonon dispersion spectrum and formation enthalpy,respectively.Pressure may promote synthesis of oP8-B2CO,which is demonstrated by calculating the relationship of formation enthalpies for oP8-B2CO via pressure.Based on bond resistance model for hardness,oP8-B2CO has a high hardness value of 47.70 GPa.Band structures calculation illustrates that all B2CO phases are semiconducting with indirect band gaps and oP8-B2CO has the widest band gap(3.540 eV)relative to tP4-B2CO(1.658 eV)and tI16-B2CO(2.988 eV).The superhardness and tunable band gap open an extensive industrial application and scientific research for B2CO compounds.