Langmuir probes are the most attractive and widely used tools for measuring plasma potential.The conventional technique to identify plasma potential from the single Langmuir probe Ⅰ-Ⅴ characteristic is the inflection point method.However,practically the inflection point is usually not clear as the knee of the Ⅰ-Ⅴ characteristic being rounded due to the space charge effect as well as contamination of the probe surface.Here we propose a novel method of measuring plasma potential,which takes advantage of a concave probe whose Ⅰ-Ⅴ characteristic shows a sharp inflection point indicating the plasma potential.