In Computer Aided Design model the shape is usually defined as a boundary representation, a cell complex of faces, edges, and vertices. The boundary representation is generated from a system of geometric constraints, with parameters as degrees of freedom. The dimensions of the boundary representation are determined by the parameters in the CAD system used to model the part, and every single parametric perturbation may generate different changes in the part model shape and dimensions. Thus, one can compute dimensional sensitivity to parameter perturbations. A "Sensitivity Analysis" method is proposed to automatically quantify the dependencies of the Key Characteristic dimensions on each of the feature parameters in a CAD model. Once the sensitivities of the feature parameters to Key Characteristic dimensions have been determined the appropriate perturbations of each parameter to cause a desired change in critical dimension can be determined. This methodology is then applied to real applications of tolerancing in mechanical assembly models to show the efficiencies of this new developed strategy. The approach can identify where specific tolerances need to be applied to a Key Control Characteristic dimension, the range of part tolerances that could be used to achieve the desired Key Product Characteristic dimension tolerances, and also if existing part tolerances make it impossible to achieve the desired Key Product Characteristic dimension tolerances. This thesis provides an explanation of a novel automated tolerance allocation process for an assembly model based on the parametric CAD sensitivity method. The objective of this research is to expose the relationship between parameters sensitivity of CAD design in mechanical assembly product and tolerance design. This exposes potential new avenues of research in how to develop standard process and methodology for geometrical dimensioning and tolerancing (GD&T) in a digital design tools environment known as Digital MockUp (DMU).