Electron cyclotron waves have cutoff densities and cannot propagate in an over-dense plasmas such as NSTX. These waves due to the mode conversion process in the upper hybrid resonance layer reduce to electron Bernstein Waves ( EBWs) which have no density limits. Therefore the EBWs are suitable for plasma heating and current drive in over dense plasmas. In this thesis the mode conversion coefficient for X-B process has been calculated. For this purpose we have solved Vlasov and Poison equations simultaneously. The method which has used to solve Vlasov equation is fourrier transformation. The motivation for using this method is that the solution of Vlasov equation in phase space becomes so oscillatory and it is impossible to be represented on a fixed numerical grid. The important problem in studying the mode conversion process is calculation of the maximum value for mode conversion coefficient that is very sensitive to plasma density fluctuation at UHR layer. This sensitivity is seen in the budden parameter scale length (L n ) at UHR layer. In order to complete mode conversion, fitting the density scale length is very important. Results of the plasma simulation in NSTX shows that for B t = 0.45 T and B p = 0.15 T, the mode conversion coefficient is maximum whe L n = 2mm.