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The disturbance resulting from the presence of a positively biased Langmuir probe at an equilibrium steady-state ion sheath boundary in the drifting plasma is simulated by the two-dimensional particle-in-cell (PIC) method in rectangular coordinates. The positive potential of the probe was found to shield electrons on the side of the probe away from the sample surface from the negative potential of the sample surface and thereby cause an increase in the electron current drawn by the probe. The drifting ions on the same side of the probe are slowed by it and thereby raising the ion density in front of the probe. The electron currents of the probe are calculated at different probe positions from the sample surface. It is found that the sheath boundary as defined by the probe current is wider than the unperturbed boundary. The widening of the sheath boundary agrees well with experimental sheath boundary measurements.