Langmuir probe is an important diagnostic tool for measuring plasma parameters. A tuned single probe has been used to diagnostic the electron energy distribution function in Ar radio frequency (13.56MHz) glow plasma. The second derivative of probe I-V characteristic was obtained after using numerical filter procedure to eliminate the statistical noise in measured I-V characteristic. A differential circuit has been designed and applied to measure the second derivative of I-V characteristic on line under the same discharge conditions. The results of the two methods are in agreement with each other.