The effects of gravity currents on longitudinal and transverse vortices between horizontal parallel plates of which temperatures change discontinuously in the lateral direction are investigated from experiments by means of a flow-visualization technique and measurement of temperature fluctuation. The flow fields are found to be classified into three different flow patterns : flow type I consists of only transverse vortex rolls, flow type II consists of mixed longitudinal and transverse vortex rolls, and flow type III consists of only longitudinal vortex rolls. These flow patterns can be classified by using the degree and flow direction of the gravity currents and Reynolds number. Gravity currents promote a formation of the flow field of mixed longitudinal and transverse vortices at low Reynolds number.