AbstractThis paper investigates the application of a floating hairpin probe for measuring the electron density in magnetized plasma. The magnetic field is systematically varied over a wide range (0.01 – 0.15 T) and its influence on the probes resonance frequency is studied. The resonance frequency in plasma displays a negative shift when the electron cyclotron frequency becomes comparable with the resonance frequency in vacuum. A general formula for calculating the electron number density in magnetized plasma is hence obtained and the formula is experimentally validated. Comparison of the electron density using the improved formula shows very good agreement with the positive ion density measured by a planar Langmuir probe in the Kamaboko‐III negative ion source (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)