Abstract: Nano-sized ZnO with small particle size，uniform size distribution and less agglomeration was synthesized by direct precipitation method under ultrasonic radiation with Zn(NO↓3)↓2 and anhydrous Na↓2CO↓3 as the raw materials，cetrimonium bromide (CTAB) as surface active agent.The product was charactered by TG-DTA，XRD and SEM，then the optimum condition of synthesis was found.Meanwhile，the prepared ZnO nano-particals were modified with oleic acid.The principle of modification was discussed simply by FT-IR method etc.and the effect of modification was analyzed by measuring activation index.