Abstract: The study of cementing displacement efficiency in directional wells has been carried out in the light of the theories of non-Newtonian fluid and two phase pipe flow mechanics.Theoretical analysis and experimental study showed that in the case of cementing in an eccentric annulus of directional wells,uncentralized casing is an important factor of displace-ment efficiency,but not the single,decisive one.Sometimes,well centralized casing did not result in high displacement efficiency and good cementing quality.Cementing displacement.may depend on the rheology of drilling muds and cement slurries.The key factors are the ratios of yield point to plastic viscosity of both drilling muds and cement slurries and the pressure gradient of displacement flow.Poor centralization of the casing may be counterbalanced by optimizing the rheological properties of drilling muds and cement slurries and the hydraulic properties of displacement flow as well.Equation(5)in this paper may be used as a mathematical mdel for optimizing these properties.In addition,it is difficult to centralize the casing in the process of cementing operation,so we would rather increase the distance between casing centralizers to make the casing between two centralizers inclined or curved and keep non-uniformity of the eccentric annulus in three dimensions than trying to centralize the casing.Under this condition,drilling muds and cement slurries are appeared to be a three dimensional flow,which can reduce and eliminate the hold-up zone,thus improving displacement in cementing.