We performed density functional theory calculations to investigate the electronic and magnetic properties of H-terminated zigzag BC2N nanoribbons (ZBC2NNRs) with the atoms arranged as B-C-N-C along zigzag lines. The ribbons can be classified into three groups according to the profiles of band structures and edge atoms: BN-BN, CC-CC and BN-CC. Among them, CC-CC and BN-CC ZBC2NNRs behave magnetic ground states. The results show that the CC-CC ZBC2NNR is an antiferromagnetic (AFM) semiconductor. Under the transverse electric field, the half-metallicity of 16-CC-CC ZBC2NNR can be achieved with electric field in the range of 0.2∼0.45 VÅ−1. Interestingly, the intrinsic half-metallicity exists in BN-CC ZBC2NNRs when the ribbon width is smaller than ∼29.2 Å. For larger ribbon width (∼33.5 Å), the system could be converted from ferromagnetic metal to half-metals at a very low critical field of E=0.02 VÅ−1. Meanwhile, it is also shown that the I–V characteristic of BN-BN ZBC2NNRs shows a negative differential resistance (NDR) effect. These ample electronic and magnetic properties might open great opportunities for BC2N materials in spintronics and nanoscale device in the future.