Abstract: Drag reducing agents are oil-soluble polymers with molecular weights ranged from several to ten million.With regard to the number of molecules,the amount of polymer contained in the oil is negligible,and has little effect on the viscosity of the oil.The drag reducing efficiency depends upon the configuration and size of the polymer molecule,and its solubility in the oil.Moreover,the efficiency also depends upon the drag reduction technology used.Experiments showed that the efficiency was increased with the amount of the drag reducing agent added within a certain limit,while the increase became less pronounced beyond that limit.The efficiency was also increased markedly with flow velocity.But its effect was not the same for different polymers.For the same pipe diameter,the efficiency was increased with increasing Reynold's number.The drag reduction efficiency was increased with increasing wall shear stress within a certain limit,but too high shear stress led to a degradation of the polymer and in turn decreased efficiency.A high water content resulted in lower solubility of polymer and lower efficiency.The drag reducing agents used in industry are polyolefins with ultra-high molecular weight at the dosage of tens ppm to give a 20% drag reduction efficiency and 15% increase in oil transported.