A measuring system is developed for diagnostics of the electron energy distribution function (EEDF) in a electrodeless low pressure rf discharge with a triple-probe system by using the probe-current modulation method. The EEDF of a nitrogen plasma in the pressure range of 10-3-10-1 Torr are obtained. The effect of rf voltage penetrating the probe sheath on the determination of the EEDF is studied both theoretically and experimentally. A new approach used to estimate the value of rf voltage in the probe sheath and to calculate the actual EED from distorted distribution recorded during experiments is presented and the experimental results are corrected by using the proposed method to obtain the true EEDF. The difference between average electron energy calculated from corrected EED and that calculated from the V-I characteristics of the probe is slight, being not greater than 5%, this result seems to give indirect evidence in favor of the procedure used to determine the actual EED and the reliability of the measuring system.