The higher order, structure preserving numerical integrators based on the modified vector fields are used to construct discretizations of separable systems. This new approach is called as modifying integrators. Modified vector fields can be used to construct highorder, structure-preserving numerical integrators for ordinary differential equations. In this thesis by using this approach the higher order symplectic numerical methods based on symplectic Euler method are obtained. Stability and consistency analysis are also studied for these new higher order numerical methods. Finally the proposed new numerical schemes applied to the separable Hamilton systems.