The modified version of the split-cantilever beam (SCB) was developed in 1995-96 and was found to be a mode-III fracture toughness measuring method involving several difficulties. The main goal of this paper is the presentation of a modified version of the SCB system (MSCB) eliminating the drawbacks of the original one and including theoretical, finite element and experimental investigations on glass fiber-reinforced composite specimens. The most important feature of the modification is that it enables the twisting of the specimen arms. First, the MSCB system is modeled as a beam under bending, shearing and torsion, and linear beam theory is used to obtain accurate solutions for the specimen compliance and the energy release rate. Second, the finite element technique, using the commercial code ANSYS is used to validate the theoretical solution. In the final part the details of the experiments are presented, which are performed by using unidirectional glass-polyester specimens. The results show good agreement between the theoretical and experimental results, and it is shown that with a careful design of the MSCB system it is possible to obtain accurate results for the mode-III toughness of composite materials.