In this study, the noise mapping simulation is executed to design an effective barrier reducing noise levels of a school site. The geographical features of the ambient site and the school buildings are modelled in detail in order to consider sound propagation, deflection, and absorption phenomena etc. The main sound source, sound power level of expressway, is estimated on the basis of measured noise levels at several points of the site. The noise mapping simulation is performed by using ENPro, environmental noise prediction program based on ISO 9613 to analysis the effectiveness of noise barrier. Consequently, the noise barrier is designed to meet an environmental noise standard and satisfy low cost and safety conditions.