We investigate the magnetism in wurtzite and zinc-blende Zn 0.875 Cr 0.125 O (wz- and zb-Zn 0.875 Cr 0.125 O) using the density-functional theory calculations. We demonstrate that the delta-doping structures are more energetically favorable than the homogeneous-doping ones. The delta-doping structure of zb-Zn 0.875 Cr 0.125 O shows robust ground state ferromagnetism (FM) with ferromagnetic stabilization energy of 575meV/Cr–Cr pair, while delta-doping structure of wz-Zn 0.875 Cr 0.125 O shows weak ground state anti-FM. We discuss the magnetic coupling in Zn 1−x Cr x O, and the origin of FM in zb-Zn 1−x Cr x O using a crystal field model. Finally, we anticipate the potential spintronics applications of the zb-Zn 1−x Cr x O.