It has long been recognized that the flow pattern，especially the velocity distribution of the fluid，is of great importance in determining the performance of the equipment using structured packing as its internals.By using three-dimensional laser Doppler velocimetry（LDV），the liquid velocity distribution of the glycerol aqueous solution was measured in a typical unit of structured packing（Mellapak 350Y）.Time averaged three-dimensional velocity distributions of the unit were measured at different flow rates.The experimental data showed that the liquid velocity profile of the single phase flow was complex because of the special geometry of the unit of structured packing.In the central region of the unit，the liquid was mixed strongly and re-distributed.It was the main cause generating the vortexes and secondary flow.