A method of robust control which was implemented by adding an external loop to a conventional feedback control system is proposed. The external loop was added to modify the control action where the information of plant/model mismatch was fed back. Robust control was carried out by tuning a filter in the external loop. One of the advantages of using the external model control structure is that the robust control was applied without changing the conventional feedback control structure Experimental investigation of a level control system was carried out. The performance of the level control system was significantly improved if the control system had the plant/model mismatch.