A model for the polar ion exosphere of Mars is developed to calculate the escape flux and density of oxygen ions and electrons through the plasma sheet of Mars at different exospheric electron temperatures (600 K--2300 K) along the magnetic field lines originating from the baropause at latitude 75 deg and longitude zero degree. The intensity of the magnetic field lines in the noon-midnight meridian plane at all latitudes are calculated by assuming that Mars has weak magnetic field of dipolar nature. To calculate the electric potential in the region of magnetic field, quasi-neutrality condition is satisfied in such a way that both the flux and density of oxygen ions are equal at every point to that of electrons. It has been found that above 2300 K, escape flux and density of O+ do not increase. The total escape rate of O+ at this temperature is ~3.5¿1024 ion/s, which shows close agreement with the observations taken by Solar Wind Plasma Instrument (TAUS) and Automatic Space Plasma Experiment with Rotating Analyzer (ASPERA) experiments in the plasma sheet of Mars. The present calculation concludes that O+ ions in the plasma sheet of Mars are mainly due to the escape of oxygen ions from the ionosphere in presence of charge separation electric field. ¿ American Geophysical Union 1995