A structural model consists of many types of finite elements, such as truss, beam, plate, shell and solid element, and so on. With the aid of commercial computer programs, field engineers comfortably use these finite elements at the same time for the modelling and analysis of real structure in their new projects. However, it is still difficult to model the connections and interfaces between different types of finite elements because of mutually ill-matched node numbers and degrees of freedom(d.o.f). To settle these problems, Many researchers studied and proposed various solution methods in literatures on FEA(Finite Element Analysis) and the use of transition elements is considered as one of the solutions. This pater presents an isoparametric formulation for three dimensional transition finite element, especially the solid-flat shell transition element. The proposed solid-flat shell transition element is composed of the solid element with 8 nodes, 3 d.o.f and the flat shell element with 4 nodes, 6 d.o.f for the simple formula derivation and the usefulness of practical applications. Basic theories for solid element and flat shell element are studied at first and a possible method for realizing the solid-flat shell transition element is suggested. On the basis of these theoretical backgrounds, the formula which calculates the stiffness matrix of the solid-flat shell transition element is derived in detail and an algorithm available for computer programming is investigated lastly.