The high Penetration of RES could have a negative effect on the power system stability by reducing its system inertia. As one of its measures, the requirements for frequency response performance of RES has been strengthened by revising the grid code for RES in the Korean power system. In this paper, it is confirmed that the frequency response of the RES can contribute even in the system inertia, and a method for quantifying its contribution on the system inertia based on its frequency response is proposed. The effectiveness of the proposed method is verified by comparing the system inertia derived with the calculated inertia constant of RES with the system inertia derived from the simulated frequency response of the RES integrated power system. The Power Simulation Simulator for Engineering (PSS/E) which is a large-scale power system analysis program was used to model Korean power system integrated with RES for the case studies.