Estimation of the Greenhouse Gas (GHG) emission abatement cost of clean development mechanism (CDM) projects is essential for forecasting the abatement cost of the cooperative approach (Article 6.2) and the sustainable mechanism (Article 6.4) of the Paris Agreement. The purpose of this paper is to compare the GHG emission abatement cost of CDM projects among four countries using the abatement cost information each country. The average abatement cost was $9.65/tCO₂ (2020 price) for all types of CDM projects across the four countries. The abatement cost of methane avoidance projects was the lowest ($4.75/tCO₂), followed by LFG ($6.49/tCO₂), small hydro ($7.67/tCO₂), biomass ($9.36/tCO₂), energy efficiency ($10.02/tCO₂), PV ($10.82/tCO₂), fuel switch ($10.92/tCO₂), and wind ($18.27/tCO₂). There were economies of scale for wind and energy efficiency CDM activities, and the higher investment needs for small hydro, PV, wind, LFG, and methane avoidance since their abatement costs are high. Economies of time existed for small hydro, LFG, methane avoidance, and energy efficiency, and there was an effect of technology development on wind, methane avoidance, and biomass. There was a correlation between abatement cost and the share of CDM activities for methane avoidance, small hydro, and PV CDM projects. A cost-effective mitigation strategy would be to proceed with projects that have higher mitigation potential at the same marginal abatement cost. Thus, at a marginal cost of $10-20/tCO₂, the effective strategy would be to proceed with LFG, energy efficiency, methane avoidance, small hydro, wind, biomass, fuel switch, and PV. At a cost of over $30/tCO₂, proceeding with LFG, wind, biomass, small hydro, fuel switching, and PV would be cost-effective.