Nonlinear effects observed in inhomogeneous plasmas are simulated using different methods. Particle in cell (PIC) method is used to simulate the conversion processes of electromagnetic modes used in heating the fusion energy equipment. Firsly, we tried to simulate conversion processes using Vlasov solver method but due to problems of parallel processing and wave creation, PIC method was used. However Vlasov solver method has been used to simulate the electrostatic and electromagnetic effects in a plasma with the characteristics of space plasmas. Vlasov solver is the method in which to study the motion of particles, the particle distribution function can be used. Vlasov equation is coupled with Maxwell's equations and their time evolution is calculated. By Fourier transformation of velocity in the distribution function in the Vlasov equation and removing the highest Fourier component via absorbing boundary conditions, the recurrence effect is strongly reduced. Particles distribution functions and electric and magnetic fields are discretized on the orthogonal grid of equal-sized cells in the position and inverted velocity. Their time evolution is evaluated using Runge-Kutta method. A series of electrostatic effects are simulated using Vlasov solver method. Firstly, the electrostatic electron Bernstein waves in a two-dimensional space is simulated. Then, the famous Bernstein-Landau paradox about the propagation and damping of electrostatic waves in magnetized and non-magnetized plasma has been investigated. Finally, the undamped Bernstein–Greene–Kruskal (BGK) modes due to nonlinear evolution of Langmuir electrostatic wave have been obtained. Also, using this method, an electromagnetic X mode wave in a magnetized plasma with the properties of space plasma is simulated. In PIC method, position and velocity of particles are defined in continuous space and fields in discrete space. Values of particles properties and fields are obtained sequentially at different times; this process starts from the initial conditions. To obtain these values, the relativistic Lorentz equation of motion is solved to obtain the position and velocity of particles along with Maxwell's equations for the fields. Ordinary-extraordinary-Bernstein (OXB) double mode conversion is investigated and simulated with using XOOPIC code in the TJ-II stellarator for the case when the input wave frequency is 28 GHz. To provide the conditions of TJ-II, nonuniform density and magnetic field profile is fully modeled. The first step of double conversion, the OX mode conversion is observed. By considering the components of the injected wave into the TJ-II and the reflected wave from UHR layer, we can easily identify O and X mode waves. Then by checking the electrical power of the injected and reflected O mode wave, the O-X mode conversion efficiency is obtained for the special case in which the angle between the injection direction and the toroidal magnetic field of stellarator is . The obtained value is 63% which is consistent with the results obtained using other simulation methods and also with experimental results. Direct X-B mode conversion in NSTX, by using XOOPIC in the case where the amplitude of the incoming wave is and frequency , is also investigated and simulated. With this range of input wave amplitude, the system is at nonlinear regime. It has been shown that the values obtained from the simulation are higher than theoretical values about 10 to 15 %. This difference could be due to nonlinear effects.