A practically simple characteristic-tensor method (CTM) is proposed for evaluation of the stress state at crack tips subjected to arbitrary loading conditions. Stress-intensity factors (SIFs) in several representative types of three-dimensional cracks (such as the circular crack, elliptical crack, and inclined circular crack) were computed by the finite-element method. Even using a relatively coarse mesh, SIFs for mixed-mode crack problems could be estimated by the CTM with high accuracy. The results show that the CTM is a practical and reliable approach to the estimation of SIFs of 3D cracks that are important to the industrial application.