Emittance of an electron beam is one of several major limitations to the performance of free electron lasers (FELs). It causes an axial velocity spread owing to the electrons_ betatron motion in undulators. In this thesis, a conditioner called "one phase solenoid conditioner" composed of two RF cavities and solenoid coil was used as an attempt to condition an electron beam for improving the performance of FELs. Analysis is presented and simulation is made for conditioning a specific electron beam. To achieve the simulation of random distribution of electrons in phase space plane, the concept of random signal (that might be performed by Fourier series) is applied. The conditioning is applied for high-energy electron beam by considering many values of energy. The procedure showed that conditioning is effective for case of high energy. The results showed that this method of conditioning is successful in reducing the randomness (increasing of correlation) producing conditioned electron beam, but it was accompanied by unavoidable growth in emittance .