In seismic design, the approximate natural period equation is defined differently for each country. Korea is currently using what is prescribed by KBC 2016. The natural period is a physical quantity related only to the stiffness and mass of the structure. If the mass is constant, the natural period is inversely proportional to the square root of the stiffness. In this study, the main objective is to evaluate the lateral force capacity inherent in the current approximate natural period equation. To grasp seismic performance, it is necessary to know the lateral resisting force of the structure. In order to know the lateral resisting capacity, the analytic model buildings were designed to have the exactly same as approximate natural period specified in the current standard, and nonlinear push-over analysis was performed. From the analysis results, the base shear coefficient and roof drift angle etc were discussed and evaluated.