Zn095Co005O films were prepared under different oxygen partial pressure P by magnetron sputtering. The effect of P on the magnetic and electrical properties was investigated. The effect of oxygen vacancy on the magnetic properties was also calculated by first-principles calculation. The experimental results indicated that Zn095Co005O films showed room-temperature ferromagnetism and high electron concentration when they were deposited under high vacuum. The ferromagnetism disappeared and the electron concentration decreased sharply when P was increased. The calculated results indicated that the energy of ferromagnetic states could be decreased by introducing oxygen vacancy in Co-doped ZnO system. The stability of ferromagnetism was determined by the distance between oxygen vacancy and Co atoms.