Biodiesel is a renewable and biodegradable alternative fuel, but its susceptibility to oxidative degradation compromises its storage stability and performance. Synthetic antioxidants are commonly used to mitigate this issue. However, there is growing interest in natural antioxidants as sustainable alternatives. This study aimed to investigate the thermal-oxidative degradation of pure biodiesel (B100) and biodiesel with coffee leaf extract (B100E) using H NMR spectroscopy as well as to correlate the iodine value (IV) with NMR spectra and monitor the formation of oxidation products throughout the degradation process. Biodiesel samples underwent accelerated oxidation at 110 °C using the Rancimat method, followed by H NMR analysis to identify oxidation products. The degradation kinetics indicated that linolenate and linoleate compounds were oxidized faster than oleate groups, leading to an increase in the number of saturated compounds. A linear correlation was found between the percentage of olefinic hydrogen atoms from the H NMR spectra and the iodine value determined by the Wijs method. The addition of coffee leaf extract effectively delayed oxidation, as evidenced by the slower appearance of oxidation products and a reduced increase in saturated compounds. These findings highlight a novel and efficient methodology for evaluating biodiesel degradation and unsaturation by combining H NMR analysis with iodine value determination. In addition to enabling a rapid, nondestructive estimation of IV, H NMR also allowed the characterization of different stages of oxidative degradation through the identification and monitoring of specific oxidation products over time.