In this work, we investigate the possibility of two-stream instability in the Venusian atmosphere leading to momentum transfer, then to the subsequent escape of hydrogen and oxygen ions from the ionosphere.We employthe hydrodynamic model and obtain the linear dispersion relation from which the two-stream instability is studied. Further, the interaction of solar wind with the ions of Venus ionosphere from which the instability sets in, has beenstudied using the data from ASPERA-4 of Venus express. The data support the fact that the two-stream instability can provide sufficient energy to accelerate ions to escape velocity of the planet and thus leave the Venusian ionosphere.