Abstract: With a three-dimensional particle image tracking technique (PIT), the instantaneous distribution of particles was non-intrusively measured in upwards large and light expanded polystyrene-liquid two phase flows in a vertical pipe. The ordered structure which was composed of particle clusters aligning in the flow direction was observed and studied. When the liquid velocity was higher than a transition velocity, particle clusters would appear, and particles would be tightly close to the pipe wall due to the strong lift force caused by liquid velocity gradient. On the other hand, particle clusters disappeared by degrees and particles were distributed closer to the pipe center with the decrease of liquid velocity. When liquid velocity was lower, particles became conglomerate horizontally. The result shows that liquid shear is the mechanism of the formation and stabilization of particle clusters. The experiment also indicated that the particle distribution structure was affected when the interaction of particles between adjacent clusters became stronger with the increase of average particle phase fraction.