A hybrid-stress Mindlin plate finite element and its sensitivity derivatives are presented. The element is triangular and has a simple nodal configuration with three corner nodes and C-o type nodal variables. The use of independent field assumptions for displacements and stresses removes the necessity for an in-plane shear correction factor. The element can effectively be used as the bending part of facet shell elements. Its simplicity and accuracy makes it ideal for large-scale analysis and design problems. The sensitivity derivatives are obtained by analytical and semi-analytical methods for the thickness and shape design variables, respectively. The well-known deficiency of the classical semi-analytical method in the shape design of flexural systems is alleviated by using a series approximation for the sensitivity derivatives and considering the higher order terms. The accuracy of the proposed formulations in computing displacements, stresses and sensitivity derivatives is verified by numerical examples. (C) 2000 Civil-Comp Ltd. and Elsevier Science Ltd. All rights reserved.