In order to accurately evaluate the contribution of mangrove forests in reducing the effects of climate change, a precise understanding of the carbon fixation capacity of mangrove trees is needed. However, a fully reliable method to estimate carbon fixation capacity has yet to be established. In this study, net carbon fixation of a representative mangrove tree in South-East Asia, Rhizophora apiculata, was estimated. It was calculated with two different procedures; the gas exchange analysis and the growth curve analysis methods. The gas exchange analysis method is based on calculated carbon values of the difference between photosynthetic absorption and respiratory emission. These two parameters were calculated by using photosynthetic rates of single-leaves in response to light and temperature and respiratory rates of trunk and branch in response to temperature. These monthly values were adjusted with monthly average measurements of light intensity and temperature to improve estimation accuracy. The value of annual net carbon fixation for 3, 4, 5 and 9 year-old forests was estimated to be 2.5-30.5 Mg C ha-1 yr-1. The values with the temperature modification increased by 9.3-21.3%, compared to those of 2.2-25.2 Mg C ha-1 yr-1 without the temperature modification. However, it was found that these estimated values were significantly higher than the results produced by the growth curve analysis method, which produced 1.1-35.2 Mg C ha-1 yr-1. Results of this study show that further work is required to improve the estimation accuracy for both the gas exchange analysis and growth curve analysis methods. There is a particular need to take into account the respiratory carbon emissions from the plant root for the former method and determination of the maximum biomass at the mature tree phase for the latter method.