This study was conducted to evaluate carbon accumulation in a mangrove plantation containing one species of Mangrove tree (Kandelia obovata Sheue, Liu & Yong) in Giao Thuy District, Nam Dinh province, Vietnam. Sixteen sample plots were established and surveyed from December 2008 to May 2010 in order to determine the amount of carbon present in the trees and forest soil. The amount of CO2 released from the forest soil was estimated once a month using a KIMOTO-HS7 gas absorption machine. The amount of carbon accumulation was calculated from estimated variables. The results show that the amount of carbon accumulated in trees and forest soil is higher than that of emitted CO2. The annual amount of carbon that is accumulated by K. obovata plantations is equ,iyalent to the amount of CO2 emitted, and this amount increases as the forest ages,. reaching the highest value in the 9 year old forest (27.138 ton/ha/year) in this present study (a CO2 equivalent of 99.596 ton/ha/year (39.35 percent). The least amount of CO2 emitted was observed in the 1-year old forest (2.207 ton/ha/year), a CO2 equivalent of 8.099 ton/ha/year (3.20 percent). Therefore, a K. obovata plantation can accumulate a large amount of carbon and function as a carbon sink and reduce the amount of CO2 in the atmosphere. This study presents scientific data that could be used to justify the implementation of mangrove plantation projects under CDM and PES programs along the Vietnamese coastline that could protect the environment, inhibit climate change, improve living standards, alleviate hunger and reduce poverty among local populations.