Abstract: Environmental study is vital for the purpose of monitoring and management of ecological status of mangrove ecosystem. Due to the difficulty to access and penetrate into the mangrove area along the coastal region, assessment of the mangrove can be studied by using remote sensing technique. In this study, the former and current condition of mangrove distribution in Penang Island, Malaysia was mapped by using Landsat-7 ETM+ and THEOS imagery. The two sets of data will be processed using supervised classification method, Maximum Likelihood Classifier in PCI Geomatica version 10.3.2 digital image processing software. The location of each mangrove sample was recorded using handheld global positioning systems during field survey to the study site. The in situ data was then being analyzed in laboratory and the actual location of mangrove area was registered into the Thailand Earth Observation System (THEOS) data to generate a geocoded map showing on distribution of mangrove over Penang Island on 2010. Confusion (error) matrix and kappa coefficient were computed to determine the agreement between the classified and reference data. The higher accuracy results of 94.40% for THEOS data and 94.00%for Landsat data indicate that the two sets of data can be used effectively to discriminate the mangrove area and monitor the mangrove changes in Penang Island. In the 8 years (2002-2008) period, mangrove cover over Penang Island had experienced an increment with area change of 12.30%.