Fluid transportation through pipelines occurs in many process industries including the petroleum industry, where crude oil and its derivatives are transported over long distances. Pipelines have long been recognized as the safest and most efficient means of transporting oil and its products. However, depending on flow conditions and pipe specifications different sources of pressure and energy loss can appear. Overcoming pressure drop, increasing the pumping efficiency and reducing energy losses have been a challenge over the past decades. Drag reducing agents (DRAs) have been found to offer such an economic relief by reducing frictional pressure drop and saving the energy required to transport fluids. These achievements have encouraged researchers to study drag reduction further so that it can be utilized better. Due to the complex characteristics of turbulence and friction, theories cannot explain the entire phenomena of drag reduction. Understanding this phenomenon and the underlying physical mechanisms are necessary for the design of the transporting system. This thesis attempts to explain the recent theories of DRAs mechanisms including background, application, and research as well as to model the effect of adding an amount of DRA to crude oil flowing through a horizontal pipe.