In order to develop a better understanding of the property, microstructure evolution and thermal history of FSW Inconel 718's, the strain, strain rate and thermal cycles need to be determined. In order to estimate the strain field of a deformed body, a displacement function needs to be determined. A 3D deformation model was developed to determine the displacement coefficients. A rectangular box created in this model deforms following a linear displacement function. Three orthogonal planes cut this deformed box, which leads to three deformed planes. The shape parameters (L, H, θ¹ and θ²) on the three orthogonal planes can be expressed as the functions of displacement coefficients. Although the displacement coefficients can not be expressed in the forms of the shape parameters symbolically, a numerical solution can be found using numerical optimization methods. The shape parameters were obtained by assuming the displacement coefficients (three cases). Then, the numerical optimization was carried out to determine the displacement coefficients. The solved displacement coefficients are the same as the assumed ones, which shows that this inverse problem can be solved, and this model is robust to determine the displacement function numerically. This model was used to estimate the strain and strain rate at the boundary of the nugget zone of Friction Stir Welding (FSW) Inconel 718. A numerical/experimental methodology was developed to estimate the thermal history in the stir zone of FSW Inconel 718.The thermocouple experiment was conducted to measure the thermal cycles in Heat Affected Zone (HAZ). Using the measured temperature in HAZ and a numerical model, the peak temperature (1039 ºC) and cooling rate (58.18 ºC/s) were determined. The microstructure in different regions was characterized and co-related with the thermal cycles. In order to understand the microstructure evolution in the stir zone, the strain rate (12.612 s-1) was estimated using the mathematical model as mentioned above. According to the estimated thermal history and strain rate, the assumption that the dynamic recrystallizaiton occurred during FSW was made. The grain size in the nugget zone affects the hardness. The relationship among the microstructure, mechanical properties, and thermal cycles was discussed.