Abstarct: Nowadays viscoelastic fluids are used in engineering sciences, chemical and polymer industries; study of the properties of these fluids is of interest for many researchers. In this study, the behavior of these fluids is modeled in conditions such that the fluid hammer phenomenon occurs in a pipe. Here, this phenomenon is called “viscoelastic fluid hammer”. The main purpose of this study is to establish the governing equations for viscoelastic transient flow in the pipe. Firstly, the properties and equations of the viscoelastic fluid are explained and then the governing equations on the fluid hammer phenomenon are rewritten generally. The system studied in this thesis consists of the valve, pipe and reservoir. Oldroyd-B equations are used as constitutive equations for the relationship of stresses. Also, the numerical method of characteristic lines is used to discretize the equations. Subsequently, the governing non-dimensional groups are computed. These groups are the Deborah, Reynolds and Mach numbers, and the viscosity ratio. To investigate the effect of the viscoelasticity of the fluid in all cases, viscoelastic fluid behavior is compared with the ideal fluid and Newtonian fluid (with viscosity similar to the viscoelastic fluid) and the results are discussed. The results of viscoelastic fluid hammer modeling showed that in a viscoelastic fluid, the effects of the line packing phenomenon at the valve are more prominent. Also, it was determined that the transient flow damping in a viscoelastic fluid is lower than in a Newtonian fluid, and so attenuation time is longer compared to a Newtonian fluid. It was argued that this is mainly related to the elastic properties of the viscoelastic fluid, such as the relaxation time constant. The elastic properties play an important role in storing the potential energy imposed on the fluid, whereas the viscous properties result in wasting of the imposed energy. These opposing actions in a viscoelastic fluid cause the damping time of the transient flow to become longer compared to a Newtonian fluid.