Coexistence of Ti vacancies and O vacancies is calculated to study ferromagnetic ordering in undoped rutile TiO₂. It shows that O vacancy (V_O) favors magnetic state in undoped TiO₂. Spin polarization induced by V_O is more localized than that induced by V_Ti, which is the reason that FM coupling of two V_O is weaker than that of two V_Ti . FM coupling of two V_Ti is strengthened as V_O is introduced. Long-range FM coupling between two separate V_Ti is mediated by electrons introduced by V_O. Magnetic moments induced by two V_O quenched as V_Ti is introduced. V_O contributes magnetic moment only if the number of V_O is more than twice that of V_Ti. It is in quantitative agreement with experimental results that V_O can promote ferromagnetic ordering, and magnetic moment increase with increase of the number of V_O.