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Abstract: As an initial effort to study the evolution of the Venus atmosphere, the influence of the solar wind density and the interplanetary magnetic field (IMF) x component (the x-axis points from Venus towards the Sun) on the O+ ion escape rate from Venus is investigated using a three-dimensional quasi-neutral hybrid (HYB-Venus) model. The HYB-Venus model is first applied to a case of the high-density (100cm−3) solar wind interaction with Venus selected from the Pioneer Venus Orbiter observations to demonstrate its capability for the study. Two sets of simulations with a wide range of solar wind densities and different IMF x components are then performed. It is found that the O+ ion escape rate increases with increasing solar wind density. The O+ ion escape rate saturates when the solar wind density becomes high (above 100cm−3). The results also suggest that the IMF x component enhances the O+ ion escape rate, given a fixed IMF component perpendicular to the x-axis. Finally, the results imply a higher ion loss rate for early-Venus, when solar conditions were dramatically different. [Copyright &y& Elsevier]