Synthetic antioxidants are commonly used for retarding rancidity in cooking oils. However, they are not effective at frying temperature and are potential carcinogens. To replace synthetic antioxidants with a natural source, which is safer and heat resistance, Pandanus amaryllifolius Roxb. (PD) and Piper sarmemtosum Roxb. (PS) were investigated. Total phenol content, antioxidant activity and synergistic effects of extracts of both leaves were determined. By comparing the results, PS leaf extract showed a higher total phenol content and antioxidant activity. So, PS was selected for further study on extraction conditions. PS extract extracted with 80 % ethanol (PSE extract) had the highest total phenol content. PS extract extracted with petroleum ether (PSL extract) possessed both highest total flavonoids and antioxidant activity. Compounds present in PSE and PSL extracts were identified using UHPLC-PDA-ESI-MS and quantified using HPLC. Seven compounds were identified as chlorogenic acid/neochlorogenic acid, caffeic acid, vitexin, ρ-courmaric acid, hydrocinnamic acid, quercetin and caffeine. The protective effects of PSE and PSL extracts on degradation of rice bran and corn oil were determined at accelerated storage and frying temperatures. In accelerated (60+3 °C), PSE extract inhibited lipid oxidation by lowering the peroxide, ρ-Anisidine, TBA and Totox values in both oils. The PSL extract did not retard lipid oxidation. However, butylated hydroxytoluene (BHT) showed a superior protective effect over PSE and PSL extracts throughout storage time. At frying (180 °C), PSE and PSL extracts had a significantly (p < 0.05) positive protective effect on both rice bran and corn oil showing a lower photometric colour, acid value, peroxide value, TBARS value and total polar compounds than the control oils. The most effective extracts were PSE 0.2 %, PSL 0.05 % and PSL 0.1 %. The Piper sarmentosum Roxb. leaf extract showed a better protective effect than BHT and therefore could be used as alternative natural antioxidants in frying oils.