Abstract: A simple diagnostic tool for density measurements in plasma with a certain spatial resolution is proposed in the this paper. It uses the emission characteristics of monopole antenna to determine the dielectic property of plasma ε=1-ƒ_p²/ƒ², with ƒ_pthe electron plasma frequency related to plasma density. We immersed a monopole antenna probe into plasma and introduced a microwave signal via a network analyzer. When the emitted power is maximized, the reflected power is minimized and there occurs a resonance. Since ε can be derived from the resonant frequency, this is actually a method to measure the absolute electron density. Validated by a comparison with the amended Langmuir double-probe method, the monopole antenna probe is valuable. In addition, it is free from the difficulties, such as fluctuation in plasma potential.