Blackberries (Rubus fruticosus L.) are a good source of antioxidants and contain appreciable levels of functional compounds. This study was carried out to evaluate fruit qualities (pH, soluble solids content, titratable acidity, and minerals), functional compounds (total phenolic content, total flavonoid content, anthocyanins, and ellagic acid), and antioxidant activity among five mutated blackberry lines over three ripening stages to examine the effect of fruit maturation on functional compounds and antioxidant activity. The content of sugars, total phenolic content, total flavonoid content, and anthocyanins (cyanidin-3-O-glucoside, cyanidin- 3-O-xyloside, cyanidin-3-O-malonylglucoside, and cyanidin-3-O-dioxalylglucoside) increased as fruit development advanced, whereas the ellagic acid content decreased as the fruit became more mature. Among the minerals tested, potassium showed the highest concentrations at all stages of development. The antioxidant activity was correlated with total phenolic content (P≤0.01), total flavonoid content (P≤0.01), and total anthocyanins (P≤0.05). The results indicate that Maple and B201 lines contained the highest anthocyanin content and exhibited greater antioxidant activity.