In the literature there is no lack of methods and rules in computing mode I and II stress intensity factors of mixed-mode problems. All these methods suffer from the fact that they are not readily available to be used in existing finite element programs without code modifications or extensive post-processing of the numerical results. As the calculation of the J-integral is now standard in FE-programs like Abaqus, Ansys, Marc etc., there is the urgent need to allow the determination of the separated K-factors of all modes in a mixed mode problem. Up to now, for this case, these general purpose programs deliver the J-integral with the simultaneous contribution of all modes. Separation of the single K-values is not a trivial task. By application of multi-point constraints, a standard tool in FE-programs, the mode extraction becomes a simple task, even for 3-D problems. No additional programming or postprocessing is necessary. The favourable convergence of the J-integral concept with respect to spatial discretization is fully maintained, so 3-D applications are not prohibitive. The proposed filter technique uses a crack closure constraint for the mode to be filtered out. The explanation for this is based on the principle of superposition and Sneddon's formulae [1] for the near crack-tip field. Numerical results obtained by the new method in plane stress problems are in good agreement with reference solutions in the literature.