An experiment combining flow visualization and temperature measurement is carried out here to study the possible stabilization and elimination of the buoyancy driven unstable longitudinal, transverse and mixed vortex flow in mixed convection of air in a bottom heated horizontal flat duct by the top plate heating. The top plate temperature is varied systematically to examine its effects on the spatial and temporal flow structures in the duct. How the top plate temperature and the Reynolds and Rayleigh numbers of the flow affect the vortex flow characteristics is investigated in detail. Specifically the experiment is conducted for the Reynolds number varying from 1 to 50, Rayleigh number from 4000 to 8000 and the non-dimensional top plate temperature from 0 to 1 at an interval of 1/8, covering a wide range of the buoyancy-to-inertia ratio. The results indicate that the top plate heating substantially stabilizes and for some cases even eliminates the longitudinal, transverse, mixed longitudinal and transverse, and irregular vortex flows induced by the buoyancy associated with the heated bottom plate of the duct. At the high top plate temperature even the entire irregular vortex flow can be eliminated and the flow becomes unidirectional in the duct. Obviously the transient velocity and temperature oscillations in the flow are completely suppressed. (C) 2004 Published by Elsevier Ltd.