Abstract: Magnetic flux ropes (MFRs) with twisted magnetic field lines are common in the solar system, most of which can be caused by magnetic reconnection or velocity shear. However, these two mechanisms have difficulties in explaining the formation of MFRs observed in the ionosphere of an unmagnetized body, such as Mars and Venus. Here we propose a new mechanism: an azimuthal component of \nabla \times \boldsymbolE can be generated by the differential compression and diffusion at different parts of the flux tube when the tube is sinking into the partially ionized ionosphere, which generates azimuthal magnetic fields with twisted magnetic field lines and finally gives rise to an ionospheric MFR. We develop a multi-fluid MHD model to study the formation of MFR in the ionospheres of Mars and Venus, and the simulation results are consistent with the observations. We also discuss the different effects of different factors, such as the sinking speed, the ion gyroradius, and the tube radius, which can be used to explain the different altitude dependences of MFRs observed at Mars and Venus.