Abstract: The perturbation phenomena in hypersonic flat plate boundary layer are investigated with electronic beam fluorescence technique in a gun wind tunnel. Mach number of free stream is about 7.8 and Reynolds number per unit length is 3.5 × 107/m. Average density distribution and fluctuation desity distribution are measured. The correlation distribution of turbulent density fluctuation and frequency spectrum distribution is obtained. Our experi mental results show that the phenomena are coexisted in order and at random in transition area.