Since energy efficiency and energy improvement are viewed as two of the most important keys of sustainable management, research on energy has become popular and captured much more attention. The current paper aimed to measure the energy efficiency of 25 countries with carbon dioxide (CO₂) emissions as well as their energy efficiency improvement. First, a data envelopment analysis (DEA) slack-based model (SBM) was employed to obtain efficiency scores. Second, the Malmquist Productivity Index (MPI) was then used to examine the improvement during the period of 2010–2017. Findings of this study showed that developed countries had a more balanced development between gross domestic product (GDP) growth and CO₂ emissions by using the same inputs as developing countries. Additionally, the findings revealed that India and China, being the two largest population countries, achieved improved energy intensity during 2010–2017; however; their energy consumption and CO₂ emissions continued to grow, leading them to being the two worst countries in terms of energy efficiency.