The standard model (SM) is one of the fundamantal theories of particle, however this model has some shortcomings in some parts of physics which led physicists to study physics beyond the standard model. One of these studied models is two Higgs Doublet Model (2HDM). In this thesis, we consider the 2HDM to calculate the branching ratio for decays. In the first chapter, we review the SM and Higgs boson. In the second chapter, we discuss the Higgs mechanism in the SM. In the third chapter, we discuss the 2HDM and effective Hamiltonian, and in chapter 4, we consider the effect of the 2HDM on decays. We calculate the partial decay width and then the branching ratio (Br) for these decays. Chapter 5 is dedicated to our numerical analysis. In this chapter a comprision is made between our results and the SM value.