Sweet potato (Ipomoea batatas L.) leaf is a natural source of phenolic compounds with strong antioxidant activity and potential utility as an antioxidant. The aim of this study was to evaluate the polyphenol composition and antioxidant activities of ethanol extracts and their various solvent-partitioned fractions (petroleum ether, ethyl acetate, and aqueous fraction) from sweet potato leaves and petioles. Seven caffeoylquinic acid (CQA) derivatives and four flavonoids were detected in sweet potato leaves by HPLC-ESI-MS. The total phenolic content (TPC) and total flavonoid content (TFC) in leaf (112.98 ± 4.14 mg gallic acid equivalent (GAE)/g of dried extract, 56.87 ± 5.69 mg rutin equivalent (RE)/g of dried extract) was more than ten times higher than in petiole (9.22 ± 2.67 mg GAE/g of dried extract, 3.81 ± 0.52 mg RE/g of dried extract). The antioxidant contents of ethyl acetate fractions increased dramatically relative to those of crude extracts for both leaves and petioles. Purification using solvent partition with ethyl acetate increased TPC and TFC of crude extracts, especially the CQA derivatives including 3,4-dicaffeoylquinic acid, 3,5-dicaffeoylquinic acid, 4,5-dicaffeoylquinic acid, and 3,4,5-tricaffeoylquinic acid. Meanwhile, the ethyl acetate fractions with the highest CQA content were associated with the highest scavenging activities towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) and higher ferric ion reducing antioxidant power (FRAP)-reducing power.Foods 2020, 9, 15 2 of 14 antioxidant power (FRAP)-reducing power was almost 6.14, 3.37, and 9.43 times higher than that of the common vegetables like spinach, broccoli, and green cabbage, respectively [12]. In addition, the cellular and in vivo pharmacological evaluation of sweet potato leaf extract exhibited a wide range of health-promoting biological activities including antioxidative, anticancer, antibacterial, antidiabetic, and anti-inflammation [9,[13][14][15]. Sweet potato leaves are thus nutritional and functional foods.Currently, 95-98% of sweet potato leaves in China are discarded as waste with low value; the remaining 2-5% are mainly used for livestock [16], which leads to a huge waste of resources and creates environmental pollution problems. However, sweet potato leaves are excellent raw materials for the isolation of phenolic compounds, which demonstrate high antioxidant activity and can be incorporated into food products as nutritional supplements [17], food preservatives [18], and/or natural antioxidants [19]. In this context, recovery of these widely available and low-cost phenolic sources from sweet potato leaves could not only improve their added value, but also solve the ecological problem that these residues cause. It is thus essential to explore extraction processes to obtain maximum yields of these substances. Fu et al. [19] found that the type of extracting solvents greatly impacts the recovery and antioxidant activities of sweet potato leaf polyphenols: 50% (v/v) acetone and 70% ethanol are efficient solvents to recover polyphenols ...