The purpose of this study is to improve the accuracy of the Building Energy Analysis Program ECO2. Energy prediction through building energy simulation in the design phase is becoming very important for building maintenance effectively and optimal energy use. The ECO2 program continues to raise controversy over the accuracy of the evaluation due to its interpretation using simple calculation methods. In this study, ECO2 is compared with a globally recognized detailed analysis program Design builder with the resulting impact difference of each input variable to improve the accuracy of the sensitivity. Design builder is assumed to be an accurate analytical value, and modeling is carried out in accordance with the Energy Saving Design Standard. Sensitivity analysis, which analyzes the change in analytical result values by varying one input variable, is conducted in two programs, respectively. The Air infiltration rate and lighting density showed an error interval in which the results did not change linearly, but changed rapidly.