International environmental issues are so important at recent year. We need a global responsibility to solve the trans boundary environmental proble . Indonesia, one of country that has a large forest in the world, hold the main role to support the environmental sustainability. The answer is how we care the world through our green ,foresfiy. This paper, "Cost Benefit Analysis in The Carbon Trading Through Clean Development Mechanism (CDM) To Support Indonesia's Reforestation," gives us the picture of Indonesia's forestry, the beneficial of forest's project and the finding of global responsibilities to make a better for mankind. Our perspective about the utility q fforest is so important to determine the decision we choose. The global environmental issues rise awareness and responsibilities, in developed and development countries. Likely global warming issues as the impact of higher carbon dioksida concentration in the atmosfer. The impact is so large among the countries. According that, each developed court.'y that has a concerned to environment, builds a commitment to reduce the emission-carbon dioksida. As developing countries, they have duties to maintain their forest, likely doing reforestation. To remind, that CDM has recognized a plantation of forest since 1990. So, we do not calculate the natural forest in carbon trading. Through CDM, forest produce certification-we called "Certified Emission Reduction" as a material in carbon trading. That certifications show how much pollution could be generated by country. Its over the rate of pollution can be tolerated by environment. Through cost benefit analysis-in the context of Indonesia, reforestation is so beneficial from economic and environmental perspective. The focus of this research is East Kalimantan in 10.000 area of forest. This paper compares the worthwhile of forest -such biodiversity, the fresh air, the clean water, woods and other forest product-with the cost doing reforestation. We use net present value to show us that reforestation activity so benefit. Time period of project is 28 years- based on other research, the reforestation project through CDM is feasible and beneficial. From the analysis-benefit and cost-the net present value of reforestation project is US $ 73.805.675. It shows us how much benefit that we earn from this activity. The potential income that would be received is US $ 2.890.500 based on the assumption that certifications could be sold in the price US $ 5.875 each carbon ton. Indonesia has a large chalice to be one of participant in CDM program. So, the cooperation between central and region is so important to create the success of this program. We need the clear law to make everything so easy to implement.