Abstract: Small angle X ray scattering (SAXS) was employed to observe the adsorption and distribution of bovine serum albumin (BSA) onto cationic spherical polyelectrolyte brush (SPB). The fitting results of SAXS data showed that the distribution of protein within brush layer increased uniformly until saturation upon increasing the BSA concentration. As pH increased, the distribution of BSA in SPB moved from the area near PS core outwards toward the solution. Moreover, time resolved SAXS was used to observe the dynamic adsorption and re dispersion of BSA onto cationic SPB in a very short time (12 s). Experimental results confirm that SAXS is a unique tool to observe in situ the dynamic distribution of proteins in SPB.