This work presents the synthesis, characterization and the thermal study of 5-n-pentadecyl-2-tert-amylphenol (alkylphenol), which was synthesized by alkylation of 3-n-pentadecylphenol. For this study were employed termogravimetry (TG), derivative termogravimetry (DTG), differential scanning calorimetry (DSC), and differential thermal analysis (DTA) techniques. Hydrogenated naphthenic oil was submitted to accelerated oxidation tests in the presence and absence of alkylphenol according to modified ASTM D-2440 method. The addition of 1% of alkylphenol to the oil has reduced the carbonyl and peroxides bands areas: 1.00-1.00 (without alkylphenol) and 0.59-0.51 (with alkylphenol) respectively. The results showed that alkylphenol reduced significantly the formation of oxidation products in the oil analyzed. TG/DTG curves showed that 5-n-pentadecyl-2-tert-amylphenol is more stable than 2,6-di-t-butyl-4-methylphenol (BHT) when analyzed in different atmospheres: 143 degrees C in air and nitrogen against 87 degrees C in air for BHT.