Overcoming the oxidation stability of biodiesel has been a significant challenge, especially after an extended storage period. To test a major factor affecting biodiesel quality, eight different conditions consisting of water at a concentration of 500 ppm and tert-butylhydroquinone (TBHQ) concentrations of 500, 1000, and 2000 ppm, in combination, were added to palm biodiesel, with no-water-added treatment as the control. Samples were kept in dark storage and air-limited at room temperature for 52 weeks with an initial carbon residue of 0.05 wt%. Every sample was periodically taken for property examination, which included the percentage of fatty acid methyl ester (FAME), iodine value (IV), kinematic viscosity (KV), acid value (AV), and oxidation stability. The properties of the samples with 500 ppm of water-added biodiesel exhibited the most significant degradation, even though oxidation stability (starting from 43.37 h) remained higher than 10.00 h after 32 weeks. The IV dropped 48.43% from 49.92 to 25.56 g I₂/100 g. The KV increased 6.14% from 4.56 to 4.84 cSt. The AV rose from 0.45 to 1.09 mg KOH/g. Biodiesel with 2000 ppm TBHQ added was stable for 22 weeks, with all properties under standard values. However, biodiesel in the same condition but with water contamination, its stability was reduced to 16 weeks.