Since the Quantum Mechanics invented, people always look for procedure to explain many-particle problems. In general, a procedure for solve Quantum Mechanics problem lead to find eigenstates, that is equivalent to block-diagonalization of Hamiltonian. But in many-particle systems finding of this eigenstates, when interaction between particles of system is not negligible , is really hard work and mostly there is no solution for the problem. To overcome this difficulty, one of the simplest procedures is simplification of the picture of the Hamiltonian. For do this, sometimes change of the basis is useful. Hence one must look for such transformation which increases the zeros of the Hamiltonian matrix although doesn’t make diagonal matrix. In this thesis, the procedure we chose was Continuous Unitary Transformation. This method at first was invented by F. J. Wegner. In this method, operation that is done is produce generator for transformation which is function of continues parameter. Using of this generator leads flow equation that is set of first order differential equations. Variation of continuous parameter is how to change original matrix to block-diagonal matrix. To produce generator it can be helpful to use quasi-particle concept. Here, particles are excitations of the system. Generator which is intended should be independent of the shape of Hamiltonian. The generator that Wegener used for its own purposes was commutator of diagonal part of Hamiltonian with the total Hamiltonian. In this thesis, chosen generator is commutator of number operator of quasi-particle and total Hamiltonian. When continuous parameter is infinite then effective Hamiltonian will be produced. The effect of resulting Hamiltonian in quasi-particle just replaces the quasi-particle in effective lattice because of conserving the number of particles. Here we are going to find effective Hamiltonian for Hubbard Hamiltonian. This Hamiltonian is divided to two parts, kinetic energy and Coulomb interaction energy. When ratio of kinetic energy to Coulomb energy is small but is not negligible so in this regime pertubativly expanding the Hamiltonian will be useful and expanding parameter is ratio of kinetic energy to Coulomb energy.with Pertubative Continuous Unitary Transformation effective Hamiltonian will be produced. In general each site of system can take all of four states from completely occupied to completely empty, but here we considered just half field states because these states are in the lowest energy states. Here existence of doubly occupied state shows that there is quasi-particle and one doubly occupied state is equivalent to one quasi-particle. In this procedure usage of Hubbard operators produce spin Hamiltonian. Spin Hamiltonian includes coupled terms interactions that represent Heisenberg interactions for different neighbors and some new kind of interactions so-called ring exchange interactions. Expressing Hamiltonian in spin representation shows that local Hilbert space decrease from four states to two different states. Key words: Quantum Mechanics, Many-body Physics, Continuous Unitary Transformation, Quasi-particle, Perturbative Expansion, Hubbard Model, Heisenberg Interactons and Ring Exchange