Abstract: Coastal development that converts mangrove forests to other uses has constantly ignored ecological services of mangrove forests. Monitoring spatiotemporal changes of mangrove forests is thus important to provide economists, ecologists, and forest managers with valuable information to improve management strategies for mangrove ecosystems. This study developed an approach to investigate spatiotemporal changes of mangrove forests in Can Gio Biosphere Reserve, South Vietnam using Landsat data during periods 1989–1996, 1996–2003, 2003–2009, and 2009–2014. The data were processed through three main steps: (1) data pre-processing to perform geometric corrections and reflectance normalization, (2) mangrove extraction using the tasselled cap transformation (TCT) and unmixing model, and (3) accuracy assessment of the mapping results. The comparisons between the mapping results and the ground reference data indicated that the overall accuracies and Kappa coefficients were generally higher than 90 % and 0.8, respectively. From 1989 to 2014, approximately 24 % of mangrove forests had been transformed to other land uses, especially aquaculture farms, while 41 % was reforested or newly planted. New insights of multi-temporal changes of mangrove forests achieved from the methods used in this study could be useful for forest mangers to evaluate successful plans for mangrove conservation and coastal development simultaneously.