Abstarct: Magneto-Rheological Fluids (MRF) are intelligent fluids which have yield stress when exposed to a magnetic field. The exposure to magnetic field will also allow to control and increase their viscosity. The MRF structure typically consists of ferromagnetic particles such as carbonyl iron powder suspended in a baxse fluid. One of the major applications of this fluids are Magneto-Rheological Dampers (MRD). Unique characteristics of MRDs including the controllable and variable damping have resulted in many damper manufacturers to conduct extensive research and developments on MRFs and their application in dampers. The aim of this study is, first, to model and parametrically investigate a twin tube magnetorheological damper. Another goal of this study is to build a magneto-fluid with good stability and also to provide a new modified non-Newtonian model to use for damper modeling. Damper modeling with numerical and analytical methods presented in this study and comparison of the mentioned methods are among the important works done in this research by which the parameters affecting the performance of damper are studied and the desired damper is designed, manufactured and tested. In the first step, in order to make the optimal fluid with suitable stability, after performing SEM and EDX experiments on three samples of carbonyl iron powder and selecting the powder with the right particle size and purity, 5 magnetorheological fluid samples were made with different compounds and stability tests were performed. Then, rheometric experiments for optimal fluid with good stability were performed at different temperatures and magnetic fields until the magnetic saturation conditions were reached.The experimental results obtained from rheology testing at different shear rate, temperature and magnetic fields were used to develop a modified non-Newtonian rheological model to predict the behavior of the optimized MR fluid and was compared to the Bingham models that are commonly used for modelling MRF flows and MRDs. Then, using rheometric data at different temperatures and fields, yield stress and parameters related to fluid behavior in the post-yield region, were obtained as a function of temperature and magnetic field. According to the obtained results, in addition to the yield stress, the fluid behavior in the post-yielding region and consequently the plastic viscosity is also a function of the magnetic field which typically is not considered in the calculation of damping force using common methods. Another advantage of the new modified model is that it can be used in CFD modeling, so that singularity does not occur and despite the dual behavior of the fluid in the post-yield region, this model predicts fluid behavior as a Continuous function. The effect of the material of the fluid and the percentage of the preservative on the fluid behavior were also investigated. The specific heat capacity and thermal conductivity of the fluid required for CFD simulations were measured experimentally. The modelling of the flow and pressure field of the optimized fluid in MR damper was conducted by implementing modified non-Newtonian and Bingham models using analytical quasi-static, analytical unsteady and Computational Fluid Dynamic (CFD) methods. The results show that neglecting factors including effect of magnetic field on MR fluid after yield, effect of wall shear stress and effect of inertia term in common modelling methods and damping force estimation of MR dampers results in considerable error. In the next step, using the apparent viscosity obtained from the new non-Newtonian modified model as a function of temperature, the effects of temperature increase due to viscous dissipation, amplitude and frequency of oscillation on yield stress, shear stress on the wall, heat flux on the wall, damping force, damping energy and equivalent damping coefficient, were evaluated in two states without a magnetic field and under a saturated magnetic field. The Reynolds number calculations in the piston gap showed that turbulent flow could not form in the range on velocities investigated. In order to reach the maximum ratio of damper force at magnetic saturation to that of without magnetic field, the Monte-Carlo algorithm and COMSOL software were used to find the optimum piston gap. The next phase is to manufacture the part and assemble the damper and assess the unit using INSTRON machine in different magnetic fields to investigate the impact of increase in magnetic field (up to saturation point) on the force and damping energy.