Abstract: The differential LDA system has been used to investigate the statistical characteristics of the turbulent boundary layer flow over a transitional rough flat plate.The characteristical quantities,such as turbulent intensity,third order moment,fourth order moment,probahilitv distrihution,auto-correlation and power spectrum,have been obtained.The experimental results expressed that the turbulent boundary layer car.be divided into subregions in the vertical direction according to the sign of higher order momentum,and the limits of divisions have been given.Meanwhile,it was found that the turbulent boundary layer flow tends to be homogeneous and isotropic owing to the roughness of the flat plate,i.e.,the distribution of probability density is close to Gussian distribution.