In vitro antioxidant activities of premature and mature mandarin peel and juice extracts were investigated for their potentials as functional food materials. Total phenolic and flavonoid content of premature and mature mandarin peel and juice was in the range of 31.20 to 94.04 mg gallic acid equivalent (GAE)/g and 0.09 to 43.99 mg quercetin equivalent (QE)/g, respectively. Among flavanone compounds, hesperidin and narirutin were identified as 76.81 and 51.35 mg/g, respectively, in the premature mandarin peel extract. Mandarin peel extracts were mostly high in in vitro antioxidant activities compared to mandarin juices. Hydrogen peroxide and hydroxyl radical scavenging activities (81.52-93.24%) of the premature mandarin peel extract were higher than DPPH and ABTS+ radical scavenging activities (24.03-30.39%). These results confirmed that the potential of premature mandarin peels as a natural antioxidant source for functional foods. The online version contains supplementary material available at 10.1007/s10068-022-01064-5.