Ultrafine NiO nanocrystallites (NCs) with grain size of 5nm have been synthesized by vacuum thermolysis method. These NCs exhibit a ferromagnetic-like behavior, and a large coercivity of 327.14kA/m exists together with a large exchange bias field of 148.48kA/m at 4.2K. A large net magnetization of 205.72μB per particle is evaluated for the 5nm NCs. The ferromagnetic-like magnetic properties are attributed to the combined effects of missing bonds and the lattice distortion.