Zn0.95Co0.05O Bulk samples were synthesized by solid state reaction method and annealed by different processes. The Zn treated samples show ferromagnetism，whereas the vacuum annealed samples do not show ferromagnetism. The XRD patterns show that there is Co impurity phase in the sample after Zn treatment at 1100℃，which is the reason of the ferromagnetism observed. At low temperature，both the positive and negative magnetoresistance （MR） effect can be observed for all the samples. We conclude that the s-d exchange_interaction_induced spin-splitting gives rise to positive MR， while the formation of bound magnetic polarons and the magnetic field suppressed spin-disorder scattering are responsible for the negative MR behavior under high field.