Summary: The problem of thermosolutal instability of a couple-stress rotating fluid is considered in the presence of magnetic field. It is found that magnetic field and couple-stress has both stabilizing and destabilizing effect under certain conditions whereas stable solute gradient and rotation have a stabilizing effect on the thermal convection when gravity increase upwards from its value . Graphs also have been plotted by giving some numerical values to the parameters. It is also found that oscillatory modes are introduced due to the presence of rotation and magnetic field and stable solute gradient which were non-existent in their absence.