Castor oil-methyl methacrylate copolymer was synthesized from castor oil(CO) and methyl methacrylate(MMA) with azodiisobutyronitrile(AIBN) as initiator and toluene as solvent under anhydrous and anaerobic conditions. The structure of copolymer was characterized by FT-IR and the relative molecular mass and its distribution of the copolymer were determined by gel permeation chromatography(GPC). Performance of the synthesized pour point depression was also evaluated. The results show that the yield(59.20%)and the average molecular weight(2.722×105) of the polymer is higher with narrow relative molecular weight distribution (PDI=1.51) under the conditions of the ratio of monomer m(CO):m(MMA)= 1:1, the initiator AIBN dosage of 0.25% at 80 ℃ for 8 h. When 0.5% (w) copolymer was added to the lubricating oil, the pour point depression is most positive(ΔSP=10 ℃).