[en] The effect of a high degree of ionization on the probe measurements of ion current has been investigated. Numerical calculation was carried out, and obtained were approximate analytic expressions for the ion current and for the region size of plasma perturbation by the probe, used in a wide pressure range both for the steady-state and decaying plasma. It is shown that effective pressure increases considerably at a high degree of ionization, the perturbation region size diminishing simultaneously. The perturbation region reaches to the flat one even at a small size of the probe. The ion current value is close to the flat probe current and is determined only by electron temperature. The ion current value is independent of the ionization degree