Measurement of turbulent flow was performed in a fully developed channel flow for both single-phase and microbubble injection conditions. A drag reduction was achieved by microbubble injection in the boundary layer. Further understanding of this phenomenon has a significant impact on energy saving. In this experiment, the Reynolds number was 5128 based on the half height of the channel. The particle image velocimetry (PIV) technique was used to obtain two-dimensional full-field velocity components in the streamwise direction near-wall normal plane. Wavelet autocorrelations were applied to the streamwise fluctuating velocity fields. By applying wavelet analysis, many of the shortcomings of Fourier analysis were overcome. The comparison of the wavelet analysis results between single-phase flow and microbubble flow indicated that the microbubbles drag reduction is a multi-scale mode, in which the small-scale fluctuations are suppressed. (c) 2006 Elsevier Ltd. All rights reserved.