Biodiesel demand is increasing all over the world. The maintenance of oxidative stability is extremely important to biodiesel market, from the point of view of the quality of the fuel as the economic amount involved. In the present work, we synthesized new antioxidant candidates for biodiesel from chemical transformations in alkyl phenols present in technical cashew nut shell liquid (t-CNSL) and, more specifically, in cardanols isolated from tCNSL. This raw material is viable, easy accessed and low cost. Five chemical transformations were performed in both t-CNSL and cardanols with yields from 50% to 92%. The starting materials and their respective derivatives were characterized by H-1 NMR, C-13 NMR and IR. Antioxidant activity and thermal stability of the products were evaluated. The induction period for pure biodiesel was 6.35 h and the addition of 0.5% (w/w) of cardanols increased to 7.37 h. Two products presented excellent antioxidant activity, epoxidized cardanols (4) and hydrolyzed cardanols (6). They increased the induction period to 15.05 h and 18.62 h, respectively. The oxygenated derivatives also showed better thermal stability than the starting materials.