Abstract: For cracked box beams, a simple exact method for determination of the stress intensity factors is proposed based on crack surface widening energy release rate. Unlike the classical crack extension energy release rate, the crack surface widening energy release rate is expressed by the G^*-integral and elementary strength theory of materials for slender cracked structures. From the present discussions, some new and exact solutions of stress intensity factors are derived for square box beam with multi-cracks. The present method is also applicable to cracked polygonal thin-walled tubes.