A three-dimensional (3D) finite element analysis on a newly developed type of coupler for glass-fibre-reinforced plastic (GRP) pipes under an internal pressure loading is presented. The stress distribution within the coupler is inhomogeneous with apparent stress concentrations located at the pipe and the coupler edges. Delamination and debonding are the main failure modes. Therefore, a fracture mechanical approach is preferred in the design of the coupler instead of a purely strength-based failure criterion evaluation. The strain-energy release rates evaluated at the critical delamination sites are used as a performance measure of the coupler. A comparative parametric study is carried out to allow for the dimensioning and shaping of the coupler as well as for the choice of an optimal stacking sequence. (C) 1998 Elsevier Science Limited. All rights reserved.