Abstract. To establish the electron energy distribution function (EEDF), the second derivative of a Langmuir probe current-voltage (I-V) characteristic is numerically integrated using the Tikhonov singular value decomposition regularized method. A comparison of the numerically intagrated EDDF and by a least-squares fitting is discussed. The used I-V characteristic is measured in an ECR plasma source using a cylindrical probe and the plasma parameters are determined by the Laframboise theory. This technique allows a rapid analysis of plasma parameters at any gas pressure. The obtained EEDF, for the case of the ECR plasma source, shows the existence of two groups of electrons with different temperatures. This result is associated with the collisional mechanism heating taking place in ECR plasma sources, where low pressure plasma is sustained by electron impact ionization of the ground state molecules or atoms by energetic electrons arising in the resonance zone.