Critical inertia can be defined as the minimum kinetic energy stored in generators that should be kept for maintaining the frequency stability of the power system. As the frequency control performance of the power system is maintained according to its control criterion, during the inertia response time frame, the expected energy imbalance can be calculated by accumulating the imbalanced power caused by the credible contingency and calculating the available inertia response by considering the allowable operating limit of the frequency. Since the frequency control criterion can be met when the available inertia response becomes larger than the expected energy imbalance, the critical inertia can be calculated by the kinetic energy of the generators, of which the available inertia response is equal to the expected energy imbalance due to the contingency. With this in mind, this paper derives the energy balance condition for the frequency stability in the inertia response time frame based on the frequency control criterion and calculates the minimum inertia, which should be kept in the generators from the energy balance condition for meeting the criterion. In addition, the effectiveness of the proposed method is verified through various case studies employing Korean power systems.