Studies of the behavior of liquid drops in a shear flow are motivated by .fundamental and practical applications in several fields of science and engineering including suspension rheology, emulsification, and mixing. However, a few people make a further investigation on the deformation of liquid drops from mechanical aspects. Through the analysis of the suffering force and deformation of a liquid drop in the discrete phase of oil-water emulsion in a shear flow between coaxial cylinders, the relation of them was deduced, mainly from mechanical aspects. A three-dimensional mechanical model of deformation of a liquid drop was propounded in a shear flow, and the relation among stretching rate, shrinkage coefficient and shear stress were established. The results show that critical shear stress can be attained if the critical stretching rate for drop breakup is determined. Therefore the stretching rate or shrinkage coefficient provides a new way to analyze the shear effect on the oil-water separation efficiency.