The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, GIc of unstitched and stitched glass/polyester composites is investigated in this paper. The GIc of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/-30, 45/-45, 60/-60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/-30, 45/-45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch2. The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on GIc is studied. Double cantilever beam (DCB) tests are carried out and the GIc is determined using modified beam theory. The GIc of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The GIc values of unstitched 45/-45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on GIc is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the GIc and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle. © 2008 Elsevier Ltd. All rights reserved.