Abstarct: This thesis try that expresses a public method for finding the solution of differential equation by using differential invariants and their usage in differential equation. Chapter 1, explain concepts and basic definitions that are needed. In chapter 2, at first define prolongation afterwards describe the prolongation of vector fields and group action with examples of them. Other section of this chapter including differential invariants with examples and the calculation method of them. As well as in chapter 3 placed differential equation and symmetry group along the method of calculating them. In chapter 4, differential equation system and symmetry group invariants are calculating by using symmetry group. In addition, reduce the order and finding the solution of differential equation order 1 and upper by integrate. At the end of chapter equation order reduction method by using Multi – parametric symmetry group are explained.