A seven flavonoids in hot water extract of immature kumquat (Citrus japonica var. margarita) were identified and quantified (mg/100g fresh fruit): 3′,5′-di-C-β-glucopyranosylphloretin (DGPP, 285.9±2.9mg/100g), acacetin 8-C-neohesperidoside (margaritene, 136.2±2.6mg/100g), acacetin 6-C-neohesperidoside (isomargaritene, 119.1±1.8mg/100g), fortunellin (acacetin 7-O-neohesperidoside, 28.5±0.7mg/100g), apigenin 8-C-neohesperidoside (16.9±0.1mg/100g), poncirin (isosakuranetin 7-O-neohesperidoside, 5.1±0.1mg/100g), and rhoifolin (apigenin 7-O-neohesperidoside, 2.0±0.1mg/100g). When immature kumquat was dried at 110 and 130°C for 0.5h, the antioxidant activity, total phenolic content and identified flavonoids increased. The UV absorbance of browning products of immature kumquat dried at 130°C for 1.5h increased dramatically, while the identified flavonoids decreased. Therefore, it was concluded that drying below 130°C for 1.0h, could release phenolic compounds, which resulted in the increasing antioxidant activity. Drying at 130°C for 1.5h, it might be due to the effect of formed browning products.