Self-assembly of polyelectrolytes onto the inner surface of a microchannel can be rather stable [1] and it may significantly change the surface charge.By proper selection of polyeletrolytes and assembling condition,the microchannels surfaces can be tuned through mono or multi-polyelectrolyte layers to eliminate the unspecific adsorption of proteins and immobilize molecules to the microchannel surfaces [2].Therefore,full understanding of the assembling process is of great importance for many microfluidic applications.In this work,the pulsed streaming potential measurement was used to monitor the self-assembly process of both anionic and cationic polyelectrolytes onto the fused silica capillaries,and its influence on the surface charge of the inner surface of the capillaries.Polyethyleneimine (PEI) was coated onto the capillaries by flushing the solutions of these polymers at various concentrations and the streaming potentials were measured along the process.The results demonstrated that 5% (w/v) PEI gave rather stable streaming potential over long periods of time,which reflected stable layers formed instantly by these polymers.As expected different streaming potentials were obtained at different pHs.For PEI the values were 0.062 V at pH 4.98,0.044 V at pH 6.50,0.042 V at pH 7.50,and 0.039V at pH 8.0 (Figure 1).For Polyacrylic Acid (PAA),the streaming potential was the same with the bare fused silica,so PAA will not assembled on silica capillarys surface.Layer-by-layer assembly of PEI and PAA alternatively was also examined,and it was found that the streaming potentials of the multilayers with the same polylelectrolyte at outmost surface were roughly same when the layer number was less than 5,about 0.036 V for PEI and -0.0551 V for PAA at the outmost layers (Figure 2).It was also observed that the solvents or the salts coexisting with the polyelectrolytes had apparent influence to the obtaining streaming potential.