International Maritime Organization (IMO) has established mandatory measures for improving the energy efficiency of ships in accordance with its reduction targets for greenhouse gas (GHG) emissions. As a result of the Paris Agreement, the maritime industry has been under tremendous pressure to reduce emissions of greenhouse gases since its inception. By 2050, the international shipping industry must make progress toward reducing greenhouse gas emissions by at least 50% from 2008 levels, according to resolutions adopted by the IMO. As part of the UN Sustainable Development Goal 13 (SDG 13), the IMO calls on governments to take immediate action to combat climate change. In order to achieve this goal, efficiency improvements must be combined with the use of alternative low-carbon fuels, as well as an increase in technical energy efficiency through the use of more efficient ship design and energy efficiency technologies (EETs). For the purpose of understanding maritime decarbonization strategy during the transition period and providing energy efficiency and elevating energy efficiency on board, legal aspects, framework, and requirements must be clearly understood for today and the future. There is a transition of the regulations for GHG emission reduction by the early implementation of the Energy Efficiency Design Index (EEDI) Phase 3 requirements for LNG carriers, general cargo ships, container ships, large LPG carriers, and cruise passenger ships. In the original schedule, it was decided to come into force in 2025. With the aim to solve the shipping industry decarbonization earlier,