The purpose of this study is to apply Landsat-8 imagery to map the mangrove forest area in Thanh Phudistrict, BenTre province. The object-based approach is used in this study including image segmentation, classification using decision tree approach and accuracy assessment. The classification results showed there were 8 classes including mangroves, Aviciennia forest, annual crops, bare land, mudflats, sand, rice, rice-shrimp with overall accuracy 97.8% (k=0.6). Mangrove forest (including Rhizophora and Aviciennia) appears along the coastline where exists the interaction between tide water and inland while other classes occupied far from the coastline. The result indicated that the possibility of using medium-resolotion images can be used to monitor the mangrove area at the district level.