Langmuir probe is an important tool to diagnose the plasma in radio frequency (rf) glow discharge. A new method has been used to obtain the electron energy distribution function in plasma by the numerical differentiation of the I-V probe characteristic.This method is based on Fourier transform and could overcome the defects of other methods. In this way, the second derivative signal of Langmuir probe I-V characteristic is obtained automatically and accurately. The mean electron energy and electron concentrations in rf glow discharge plasma in silane were obtained by Langmuir probe.