Abstract: The fastener flexibility is one of the major concerns in the strength analysis and design of the mechanical fastened joints. A method for determining the fastener flexibility, based on the theory of beam on elastic foundation, has been proposed in the ESDU (Engineering Sciences Data Unit) Datasheet 98012. In this paper, some equations and boundary conditions of ESDU’s approach are modified and their usage is extended to both single and double-lap joints in which the joint plates may be of different materials and/or different thickness. A series of tests were carried out to measure the tensile flexibilities of single-bolt and single-lap metal to composite joints. Three dimensional finite element models of the specimens were established as well. The predicted results of the specimen flexibilities using the modified method show good agreements when compared with those from experiments and finite element analysis. The effects of certain parameters, such as the foundation stiffness, the fastener head constraint and the equivalent plate shearing area, on joint flexibility were also investigated.