Abstract: To reveal the argon plasma characteristics within the entire region of an electron cyclotron resonance (ECR) ion source, the plasma parameters were diagnosed using a bended Langmuir probe with the filament axis perpendicular to the diagnosing plane. Experiments indicate that, with a gas volume flow rate and incident microwave power of 4 sccm and 8.8 W, respectively, the gas was ionized to form plasma with a luminous ring. When the incident microwave power was above 27 W, the luminous ring was converted to a bright column, the dark area near its axis was narrowed, and the microwave power absorbing efficiency was increased. This indicates that there was a mode transition phenomenon in this ECR ion source when the microwave power increased. The diagnosis shows that, at an incident microwave power of 17.4 W, the diagnosed electron temperature and ion density were below 8 eV and 3 × 10¹⁷ m⁻³, respectively, while at incident microwave power levels of 30 W and 40 W, the maximum electron temperature and ion density were above 11 eV and 6.8 × 10¹⁷ m⁻³, respectively. Confined by magnetic mirrors, the higher density plasma region had a bow shape, which coincided with the magnetic field lines but deviated from the ECR layer.