Abstract: Castor oil based atom transfer radical polymerization (ATRP) initiator was synthesized by one-pot reaction of 2-bromoisobutyryl bromide with hydroxyl group in castor oil under hydroxyl-to-bromide molar ratio of 1:1 and characterized by means of FT-IR, 1H NMR and 13C NMR. The results showed that the esterification for the synthesis of castor oil based ATRP initiator is prompt and highly efficient. In FT-IR spectrum, the conversion of hydroxyl group to ester group resulted in the decrease in the intensity of peak at 3500 cm-1 corresponding to hydroxyl group and increase of peak at 1740 cm-1 corresponding to ester group. This indicated that 2-bromoisobutyryl group was grafted on castor oil successfully. Subsequently, the castor oil based ATRP initiator was used to trigger the ATRP of methyl methacrylate (MMA) and styrene (St) under the condition of feeding molar ratio (monomer/initiator/CuBr/ligand) of 100:1:0.5:1 and reaction temperature 60 ℃ for MMA and 80 ℃ for St, respectively. GPC curves and spectra of 1H NMR and 13C NMR showed that polymerization process of MMA, as well as St, was highly efficient and well controlled.