We have investigated the exchange coupling between an epitaxial Ni ͑50 Å͒ film with an out-of-plane magnetic easy axis and a NiO ͑600 Å͒ film by polar magneto-optic Kerr-effect measurements. The temperature dependences of exchange field H E for both as-deposited and field-cooled states exhibit the same blocking temperature T B ϳ130°C. The exchange field H E for the field-cooled state is lower than that for the as-deposited state. The hard-axis in-plane loop shows a much smaller value of H E . No coercivity enhancement is observed. The data are suggestive of linear coupling across the ferromagnet/antiferromagnet interface.