Abstract: Based on the refined triangular Mindlin plate elements developed by Chen, a new triangular composite stiffened plate element with 21 degrees of freedom has been proposed. The formulation of the element considers the coupling effects between in-plane bending and torsion as well as transverse shear effects for the stiffeners. Because displacement compatibility between the stiffeners and the plate is well maintained by using the same shape functions for both elements, and additional degrees of rotations of the stiffeners are introduced, the present element can describe the deformation characteristics of the plate and stiffeners accurately, compared with existing stiffened plate elements. In addition, the number and orientation of the stiffeners in the element can be defined by users, thus the meshing procedure is rather convenient. The numerical results indicate its effectiveness, especially, for analyzing the plate stiffened by thick beams. Finally, some discussions for the effects of boundary conditions and loading cases upon the deflections of composite isogrid structure (AGS) are also carried out.