In this study, total phenolic and flavonoid contents, antioxidant capacity, and phenolic compositions of Castanopsis phuthoensis and Castanopsis grandicicatricata (Fagaceae family) were investigated. It was found that bark extracts were rich of phenolic contents, whereas leaf extracts were abundant of flavonoids. The total phenolics varied from 11.20 to 35.47 mg gallic acid equivalent g-1 dry weight (DW), and the total flavonoids were from 2.24 to 12.55 mg rutin equivalent g-1 DW. The results of antioxidant activity evaluation showed that the DPPH (1,1- diphenyl-2-picryl-hydrazyl) radical scavenging activity of the free phenolic extracts were higher than the bound phenolic extracts. Regarding the reducing power and β-carotene bleaching assays, the free phenolic extracts showed remarkably strong antioxidant capacity that were similar to the levels of the standard BHT (dibutyl hydroxytoluene) did. It could be concluded that free phenolic extracts were more effective in antioxidant activities than bound phenolic extracts. Highly significant correlations were observed between phenolic contents and antioxidant activities (0.813 for DPPH and 0.841 for reducing power). By HPLC analysis, seven phenolic acids were detected including gallic, p-hydroxybenzoic, vanillic, sinapic, p-coumaric, ellagic acids, and vanillin. Of which, gallic, ellagic, and sinapic acids were the most abundant compounds in the two species. The results suggest C. phuthoensis and C. grandicicatricata contain rich sources of natural antioxidants and phenolic compounds which are probably considered in pharmaceutical use