Hybrid code simulations of the MHD scale Kelvin-Helmholtz (K-H) instability have been conducted. In contrast to the homogeneous background case where the large-scale ion mixing across the shear layer occurs inside the vortex obeying the hydrodynamic similarity law, the ion mixing is found to be reduced when the background magnetic field is inhomogeneous. This stabilization effect is twofold: it is an ion kinetic effect which reduces the saturation vortex size and so the mixing length when the shear layer width and the wavelength of the growing mode are small. The reduction effect still works even when the shear layer width/wavelength are large and a well defined vortical motion as in the fluid model is evident. The mixing is suppressed for this case because the ions on the lower magnetic field (magnetosheath) side cannot enter within the vortex island: The rolled-up magnetopause current sheet block them out electrostatically. The geophysical implication of the present result is that the K-H mixing alone may not be as efficient as to produce the large-scale mixing blob of the low-latitude boundary layer (LLBL) when the magnetic field intensity is highly different across the magnetopause. ¿ American Geophysical Union 1995