Abstract: The stiffness matrix of a mechanism was defined on the basis of the Hessian matrix of its potential energy, and a new continuous stiffness mapping model of the modified Delta mechanism was established. Different from the traditional models, the proposed model takes into account the influences of not only stiffness of the active/passive hinge and gravities factors on the mechanism stiffness, but also the change of the mechanism configuration and all parts elastic deformation under the constraint force. It is thus more general and exact than any other stiffness models. Finally, the characteristic index λ used to estimate the mechanism stiffness was defined by combining the Rayleigh quotient of the stiffness matrix. The stiffness characteristics were analyzed and discussed according to the distribution of stiffness index λ at both different positions in the workspace and different applied forces.