Biodiesel is an alternative fuel for diesel machines, forming from vegetable oil, animal fat, and cooking oil. The use of biodiesel increases with the enactment of Indonesian Ministerial Regulation of Energy and Mineral Resources No. 12/2015, which requires the use of biodiesel (B100) by 20% in motorized vehicles, starting from January 2016. Palm Fatty Acid Distillate (PFAD) is a by-product of the vegetable oil industry, that can be converted into biodiesel. PFAD used in this research was obtained from the Palm Oil Plantation Research Center of North Sumatra, with free fatty acid content of 95.2 %mol. PFAD, which was converted to biodiesel through an esterification reaction is 99.6%, with molar ratio of PFAD:Methanol of 1: 9 and addition of sulfuric acid catalyst of 0.6% v/v methanol. Analysis of oxidative stability in biodiesel was carried out using Rancimat method. Biodiesel stored for approximately one month has an Induction Period (IP) of 5.25 hours, decreases from 6.15 hours without storage. Storage time for biodiesel can be extended by the addition of antioxidant X. In terms of economy, the optimum antioxidant X concentration added to biodiesel is 0.06 ppm. Biodiesel stored for approximately one month with addition of 0.06 ppm antioxidant X, has an Induction Period (IP) of 6.34 hours. Reaction rate constant of antioxidant X in biodiesel at temperature of 110°C is 0.0042 m−1 and the critical concentration of antioxidant X at 110°C is 7.618 × 10−6 M.