The Kyoto Protocol implemented the first international top-down mechanisms and provided mitigation incentives for both Annex B and non-Annex B countries. An assessment of the Kyoto Protocol would contribute to the fundamental discussion on designing future mitigation mechanisms under Article 6 of the Paris Agreement, while the use of a top-down approach that includes an emission trading scheme appears important for achieving the 2 degrees C target. This paper summarizes the existing literature and quantifies the effects of greenhouse gas (GHG) emission reduction impacts from the Kyoto Protocol using the Stochastic Impacts by Regression on Population, Affluence and Technology (STIRPAT) approach for Annex B countries and 'additionality' assessment of Clean Development Mechanisms (CDM) projects for non-Annex B countries. We found that the Kyoto Protocol brought about 951 Mt CO(2)e of real emission reductions in all over the world, mainly from implementing non-energy related GHG emission reduction projects in non-Annex B countries. For the Annex B countries, 76 Mt CO(2)e of mitigation occurred during the preparation stage of the first commitment period (2005-2007), but no further effects were observed during the first commitment period (2008-2012). The following important lessons were learned from the implementation of the Kyoto Protocol: (1) insufficient emission caps did not provide any mitigation incentives; rather, they resulted in perverse effects amounting to 12 Mt CO(2)e, which increased emissions in certain Annex B countries with economies in transition; and (2) since 42% of energy-related projects in non-Annex B countries were assessed as non-additional projects, more attention needs to be paid to the design of international cooperation mechanisms.