The large-scale coherent structure in a density-stratified turbulent flow under strongly unstable conditions is investigated experimentally by using some conditional sampling techniques. An array of the large-scale longitudinal vortica rolls can be recognized by using the new conditional sampling technique, that is, the thermal-plume (TP) detecting one to single out the hot or cold thermal plume at the boundary of ordered turbulent rolls. The conditional sampling measurements provide the detailed knowledge of the fine structure of turbulence and the mechanics of turbulent heat and momentum transfers.