The apparent viscosity of dilute and semidilute xanthan solutions flowing through small cylindrical pores, under conditions where there is no adsorption effect, were accurately determined in Newtonian and shear‐thinning regimes. The analysis of the results using a two‐fluid flow model shows that the hydrodynamic thickness of the depletion layer near the pore wall is closely related to the equivalent rod length of the macromolecule, but seems to be insensitive to the persistence length. The hydrodynamic thickness does not depend on polymer concentration as expected for a semirigid polymer and decreases slightly as shear rate increases.