This study investigated total phenolic contents, antioxidant activities, and antidiabetic activities in flesh, seed and leaf extracts, and the fermented juices of Garcinia schomburgkiana Pierre (Madan). Seed extract showed the highest total phenolic content, radical scavenging activity and amylase inhibitory activity, whereas the leaf extract had the strongest ferrous ion chelating activity and inhibitory activities on glucosidase and lipase. The very strong positive correlation between total phenolic content and radical scavenging activity, and between ferrous ion chelating and glucosidase inhibitory activities were significantly found (r = 1.000 and 0.998 at P<0.05, respectively). Moreover, the Madan juice revealed the highest total phenolic content at 72 hour fermentation. However, production of radical scavenging and ferrous ion chelating activities of fermented juices increased at the start of fermentation. Additionally, amylase inhibitory activity increased during 24 hour fermentation, whereas glucosidase and lipase inhibitory activity reached maximal levels during 17 day and 48 hour fermentation, respectively. The total phenolic contents of the fermented juices were very strongly negatively correlated with radical scavenging activity, and ferrous ion chelating activity, significantly (r = -0.869 and -0.937 at P<0.05, respectively). In addition, the very strong positive correlation between radical scavenging activity and ferrous ion chelating activity was significantly found (r = 0.804, P<0.05). Our results indicated that G. schomburgkiana Pierre is a potential nutraceutical source, and its fermented juice can be further improved as a healthy fruit drink.