Fluid flow patterns on a distillation tray played an important role in the tray efficiency. A new technique for measuring fluid flow patterns on a distillation tray was introduced. The new technique involved observing a step input of a liquid tracer of higher temperature while keeping a record with a thermal infrared camera, and the principle for measurement was also deduced. Experiments were carried out in a 1200-mm-diameter simulation column using two types of trays with one and two hanging downcomers. With the new technique, the detailed picture of the residence time distribution on a multiple downcomer tray was obtained. The results showed that the new technique was convenient, accurate, and not contaminated to measure the residence time distribution of the tray. A stagnant region existed in the multiple downcomer tray. The size of the stagnant region decreased with a decrease of the liquid rate and bubbling of air. The initial velocity of liquid entering the tray determined the flow pattern of the tray.