Three methods (eco-efficiency, environmental priority strategy (EPS) system, and certified emission reduction (CER) price) have been proposed to support the decision-making processes that simultaneously consider cost and CO2 emission in acquiring an economical and environment-friendly design. However, which method is most reasonable is still being debated. This study was conducted to determine the differences in the results among three methods that simultaneously consider cost and CO2 emission. Towards this end, the case study was conducted as follows: (i) calculating the costs and CO2 emissions of nine building structural design alternatives; (ii) identifying the design alternative priority by applying the costs and CO2 emissions of the nine alternatives to the three decision-making methods; and (iii) comparing and analyzing the causes of differences among the results. Results of this study showed that the three methods differed in the influence ratio of CO2 emission on the decision-making results. The influence ratio of CO2 emission in the eco-efficiency-based method was 29.1 similar to 78.4% while those in the EPS system and CER-price-based method were 18.8 similar to 23.5% and 1.8 similar to 2.3%, respectively. Due to such differences, the design alternative priorities proposed by the three methods varied. (C) 2014 Elsevier B.V. All rights reserved.